Hello JHC.
You are definitely on the right track. I for one can tell you that by using the tube buffer as a "unity-gain-impedance-equalizer", this is exactly what you need if you understand the technical parameters for its use as an intermediate device.
Aside from that, just try one and you won't need to understand anything other than the beautiful music you are experiencing as a result. This is the perfect intermediate solution to find balance between digital and analog. The tube buffer is essentially a hybrid device which gives you analog flavor by correcting and removing the digital artifacts which make the music hard and edgy by nature of the digital waveform.
Of course, it is critical to have a very stable transport to ensure low enough jitter rates, or nothing else will even matter after that if your transport is substandard. Always start with a high-end transport. Then build upon that. A streamer is a different story.
The greatest DAC in the world will not give you the sound quality you seek if the jitter rate is not optimal. This simply means that your music will never be properly focused upon the bits being read by the laser pick-up as they are transferred to the external DAC via coax cable or fiber optics. By the way, use the coax cable due to the fact that fiber optic cable actually is part of the jitter problem itself. The coax is a better choice for stability.
The more filth that gets onto your eyeglasses, the harder it is for you to focus without distortion to your sight. With clear lenses, you see everything perfectly without confusion.
The Zrock3 is also a device which is essentially a tube buffer acting in very much the same manner. This is what makes these devices so perfect for this use. There is a reason why the music takes on a corrected re-alignment to the sound quality by using buffering in this manner. It is very effective.
By using a tube buffer, your digital music will inherently take on an analog sound quality which nothing else can even approach. That being the correction factor which you are implementing here. In my opinion, the Zrock3 does a fine job of doing this, but considering the cost of this device, and what you can actually build in regards to a proper high-end tube buffer if you have that ability, you will achieve the goal you seek at a much lower cost-to-gain ratio.
Another benefit is that you control the waiting list for getting your device built.

With the X10D board, you have the ability to fine tune to taste by means of EASY tube / capacitor rolling. Not so much with the Zrock3. What you DO get with the Zrock3 is tuning flexibility by means of actual gain input in which the tonal balance can be adjusted with refined precision. The Zrock3 is an excellent device with my full recommendation if you choose to go that route.
If you use a high-end preamplifier along with a high quality tube buffer such as the newer MOFI X10D design, then you will essentially have the same degree of control. There are things that the Zrock3 can do which does make it appealing as a stand alone device. However, I have found that by reaching a high level of source and amplification quality in combination with a properly balanced acoustic setting, I just don't find myself using the Zrock3 anymore since it isn't necessary in my case.
I just invested $1800 in something that now collects dust. My system simply does not require it. The X10d is very effective in doing what it is designed to do.
This is not to say that there is no need at all for using the Zrock3. I just don't find it necessary to use both. My tube buffer is part of the DAC, this for which makes this DAC so special for how it portrays music in the way it should be heard if your focus is upon a natural analog sound quality.
I simply cannot tolerate digital music without a tube buffer. I would be investing in a $30,000 vinyl rig otherwise in order to get the sound I am seeking. I am not interested in fussing with the complexities of dealing with high-end vinyl again. Been there, done that. Just too much hassle for me to deal with for the actual difference in gain that I will acquire by going that route.
A well engineered tube buffer doesn't cost anywhere near that. As to the comparison of sound perspective between the two, well that is really a deep rabbit hole which you may fall so deep into that you never find your way back out. This difference is subjective to the listener, and most definitely is subjective by means of all other factors within your audio system along with how good your room acoustics actually are. Just remember, your room acoustics account for at least fifty percent of what you hear.
If you don't have a tube buffer, then the Zrock3 is a good choice to make. If you need to choose between one or the other, I absolutely recommend the X10D tube buffer stage directly between the CD player and the preamplifier/amplifier.
When digital first came out back in the early eighties, The British company which developed the original Musical Fidelity X10 tube buffer knew exactly what was wrong with digital. They were the ones to first offer a cure for the harsh jagged sound of digital when it was in the early stages. That became an extremely effective device which gained huge popularity in those days simply because it works to correct the things that makes digital harsh. It is still around today because it is so effective and popular. The end result is a refined sense of music which emulates the true analog sound the way that the original recording sounded before the digital transfer process.
From my perspective, as you may know, I have recently acquired a new Zrock3 with ALL upgrades and options which take it to the highest level. After using that device quite extensively between the DAC and torii amplifier, I have grown quite familiar to its ability as compared to using a tube buffer with, and without it.
My take on this is as follows: I highly recommend the Zrock3 for smoothing out bad digital recordings for which that is its purpose.
But also, I have come to realize that a good quality tube buffer stage such as the newer upgraded MOFI X10D tube buffer does essentially the same thing, and absolutely just as well in my case. You can benefit by using both of these devices simultaneously, but recently since my upgrade to the Zenith amplifier, I have discovered that the Zrock3 is no longer needed at all in my system.
As a result of my new amplifier, the balance between my source and amplifier is near perfect, to the degree that nothing else is required for optimal sound quality.
Let me give you an example as to why my latest comprehension of the music quality now differs from just a couple of months ago.
What I discovered actually is proof of the importance of system synergy, and exactly why fine tuning this system synergy is indeed so critical to the overall outcome for which you experience. Obviously, it all focuses to one very important thing, that being the less restrictive means for which the amplifier and your source communicate with each other within the circuit path. This sums everything up by two words being: IMPEDANCE MATCHING!
When there is proper synergy, everything falls into place naturally.
I am not going to go beyond that explanation in technical terms. Just know that this is the primary factor and it is absolutely critical.
I am listening to CD's now that I thought absolutely were dependent upon the Zrock3 to be decent for enjoyment before I modified and upgraded the amplifier design. I can tell you that these CD titles just seemed lifeless and dull when I had the stock amplifier in its aging form. With the Zrock3 in place, fine tuned to the right degree of tonal balance, the improvement by using the stock amplifier was substantial, with the proper degree of enhancement to the sound structure as a result.
That in combination with my DAC using the Musical Fidelity X10D buffer stage, the three of these devices in unison seemed well balanced, and quite acceptable within the outcome of the music presentation.
As I said, before the amplifier upgrades, I needed the Zrock3. Now, it is a completely different outcome. What I extracted from the Zenith upgrades was all it took to bring it all together. That alone made a profound difference to say the least.
I listen to those same CD's now and what I hear is on a completely higher level of clarity, definition, and tonal balance. In perspective, that is what was missing in my system all along. I just had to extract it.
The problem is not always as it would seem. The obvious is not always so obvious. Once corrected, you will know very well what has been eluding you.
It wasn't so much the CD being poorly recorded. The sound quality just wasn't revealed to the full extent in order to perceive what was really there in that recording. I would have never believed this was possible had I not proved it to myself. The fact is, my experience is solid proof why everything in the signal chain matters completely as to how this communication path is to be effective.
Remember the computer term: GIGO? ( "GARBAGE IN GARBAGE OUT" )...Can't place things in a more simplistic manner for which describes the outcome.
I wonder how many people would tolerate going back to the DOS interface...wouldn't be so many people online I would imagine.
Anyway, the path you choose will be effective either way. It really is subjective to your situation and what you really need.
Let me ask you this, do you have the skills necessary to assemble a tube buffer from a blank PCB? If you know how to solder correctly, then it is actually quite easy.
The most important thing is using quality parts which absolutely are critical to the sound quality. High end capacitors and the right choice of tubes make a huge difference in the outcome. And of course, the power supply is just as critical. A high quality R-core transformer is a must!
If you are interested, follow along with my project build on my thread in the acoustic section. I am now just starting that upgraded build of the new high-end MOFI tube buffer stage using premium quality parts.
I will be showing you in detail step-by-step how to build this tube stage. If you do it yourself, it will be very inexpensive to build, but the sound quality will be on par with any esoteric component out there, regardless of price. Price is not a factor of quality. Status does not make my system sound good, properly engineered components built with common sense does.
I am going out of my way by doing all of this so that people such as yourself can actually see that you can have the best without paying a fortune to obtain it. Take that as it is. If it works in my favor, then you too should reap the benefits as well.
We are seeking solutions here which benefit our music systems, not to benefit someone else's bank account.
If the cost factor is definitely a strong consideration for you while trying to maintain a balance of quality and performance, then DIY is definitely your answer here. I would go with the MOFI X10d design myself. But that is a decision you will have to make.
No matter what, I think you will be pleased with the results.
Just my opinion, but that is something to consider.
Hope this somehow made sense and actually helped you to get a better understanding based upon my experience using both devices.
So check out my build on occasion. If you have questions, I can try to help you. I try to be as thorough as possible in my explanations. I am sure that I can help you build your own tube buffer which I guarantee you will have excellent results.
Just pay attention to detail and do NOT deviate from what is important. If you build it correctly, you will be immensely rewarded with the results.
Okay, I will be around.
Have a good evening.