I ran my MKIV without the CSP3 for months to learn it, but over the last month, I have had a chance for serious experimenting with and without the CSP3.
My Torii MKIV has roughly 650 hours. It is finally past burnin and into refinement. After 300-350, it lost the ups and downs, followed by the occasional leap, but something happened lately that feels conclusive….an opening that feels relatively complete.
My CSP3 has maybe 350, so who knows with it, it could well need more time.
With both, they really come alive after the wax caps are warm.Right out of the box, I really enjoyed the
stock CSP3 with my Torii MKIII. Not exactly transparent, but really excellent… involving and compelling…dynamics, body, solidity, clarity, bass tightening, and a special sort of spaciousness and blackness from which it all emerges.
As my MKIV arrived, I sent the CSP3 back to be modded to accept 6-type tubes other than 6N1P and 6BQ7A in the power positions, and to be recapped with Jupiters. Mine is #3 and most had no issues with various tubes, so I am guessing the mod I got was incorporated in general.
A few hundred hours on the new caps, I have been exploring the "new" CSP3, now with the MKIV. In
some ways I preferred the stock caps with the MKIII, a contrasting signature with an intriguing quality, open and characterful, and I think with a bit better definition down low.
The Jupiters have seductive qualities also, with amazingly refined warmth and definition. This is a more revealing, organic, and transparent feeling signature….quite similar to the MKIV actually, with potential for amazing micro detail, spaciousness, harmonics and textures…edges are soft and sweet with this fine detail contributing to a natural musicality.
The Jupiters in these simple circuits are big players toward the similar sound of the CSP3 and the MKIV, and though less contrasting, it can facilitate a pretty pure portrayal of the music. Since I work toward the revealing side when tubing gear, to preserve micro detail and associated textures, harmonics, and ambience….with these very distinctive and revealing caps, openness can get a touch tricky...
Like the MKIV, the Jupitered CSP3 reflects different tubes very clearly. Both can be tuned/tubed further toward warmth, but also to extreme transparency. I find I can go over the top either way, but they do these really well offering flexibility for tastes and to adapt to different system/rooms. Innate musicality, bridging warmth and amazing detail indicates the extreme care of circuit voicings. I think Steve's choices were really good.
When set up toward the detail side, since they both amps allow exceptional dynamics and clarity, things can get too intense for me! But micro detail and micro dynamics are critical to me, and balancing just enough of the "revealing" with natural dynamics and impeccable tonal balance is the challenge with each tube sound so clearly revealed. Finally, for spacious beauty, this revealing nature is necessary for a synergistic realism that would not otherwise be achievable.
Both the MKIV alone, and MKIV with the jupitered CSP3 can bring out amazing sound, but in this system/room, there were sacrifices and benefits to each…and what the jupitered CSP3 brings to the MKIV is so similar to what the MKIV already does, the addition is less obvious, there is less contrast to work with. The CSP benefits are there, especially initial gain setup, and gain riding being very useful….but the CSP3 sound traits….deep black, open spaciousness, micro dynamics, enhanced resolution and definition, weight and body…the MKIV is already exceptional in these areas.…for me a step up in the Torii line….almost like it has a mini CSP hidden inside. However, the CSP3 can and will take it further and sound beautiful.
The real catcher for me has been this: until the last two days I have not been able to conclusively say the CSP3 makes my MKIV better…. The MKIV reconstructive feedback circuit has shown me that my DAC is incredible at micro detail retrieval. With reconstructive feedback switched in, the MKIV pulls this very subtle information beautifully from the Tranquility DAC. This is revelatory stuff, and through most of my explorations, the CSP3 merged aspects of this into its signature, integrating it, in effect masking it...When I say subtle information, I don't mean subtle in the presentation. The expressions of very fine detail on top of "normal" detail are critical to the completion of everything that is music...the textures; attacks; natural differentiation of sounds…timbre; trails; sounds of the room; air; delicacy.... and all the many things that complete detail reveals. It shows up everywhere, but the edges of, and spaces around sounds are so beautiful.
With different preferences, or if it were less there from the source, these thoughts may not even come up, as the CSP3 does sound amazing. The Tranquility as I have it set up is special though and since the well set up MKIV actually retrieves this information, it seems easier to degrade the combination than to improve it, at least in the micro realm.
I actually remember that the Zstage introduced some of the above traits with my Rega Apollo by putting tube beauty after a lesser front end. So though this all depends on so many things, the MKIV's ability to allow very interesting musical information from a good source is a notable aspect of the question.
But back to my desire to get the jupiter CSP3 and MKIV combination better than the MKIV alone! I finally found a synergy with the CSP3, now passing most of this information with relative transparency. Before these sounds were there, but not as clearly as in the presentation of the MKIV alone. At the same time, the CSP3 sound was compelling in different ways, offering increased dynamics, body, and definition, more obviously mids down.
On the other hand, the MKIV alone is not lacking in these qualities and has a more spacious and relaxed sound alone. Thus the dilemma for me. No doubt, the CSP3/MKIV were exceptional together, just not ideal in my personal pursuits of the last few percent.
I should mention here, that the 6N1P just could not pull the subtleties. It is very good at its own sound, particularly with clear bass and warm, but clear feeling mids, but its forte is not subtle fine detail as far as I can discern. This led me to exploring tubing that finally contributed to making my MKIV and CSP3 revealing of fine detail together, and thus similar, both tuned for revealing transparency within a slightly warm and musical body.
Now The masking is barely discernible. With matched volume, with and without the CSP3, the sound is quite similar…but the CSP3 does do CSP things....there is a bit more liquidity, sweet definition, body and dynamics….
Is this change from burnin, or just the right tubes, cable and foot synergy? I would say all, but I am thinking burnin is a big player.
The other big player that was clear was switching power cables. I have a DIY Neotech 10 AWG with Neotech gold ends, that I was the most transparent and dynamic cable in the Torii MKIV (at least with the current synergy) and to the CSP3 I had a 10# VHAudio DIY with Gold furutech ends (warmer, but revealing also). A week or so ago I switched these cables, thinking maybe the Neotech would make the CSP3 more transparent without losing body and dynamics. It did, but a few days later, I was still struggling.
Then I thought of a PI Audio cable (quite neutral and transparent, but musical). I like it a lot in the Torii MKIII, so I pirated it and put it in the MKIV. Helped some, but not quite there…a little lean. On a whim I switched the Neotech back to the MKIV and the PI Audio to the CSP3. Wow, clarity, body, dynamics, the best yet. Next I put some Amperex Orange PQ gold pins in the MKIV. Another little leap? Though I found 6922s more difficult in the MKIII, tending to chilly, in this combination, and in the MKIV, very nice! The MKIV handles inputs differently than the MKIII. I have been using the tall bottle Siemens PCC88/7DJ8 and still probably will end up preferring this. I do tend toward 7DJ8s....More experimenting!

Then it took a few more days, with more tube and fuse exploration to get things straight, to this place of very sweet sound without masking. And I don't think it was the tubes and fuses. I am back to a few that kept rising to the top during this saga.
Bottom line? Though it takes the right cables, tubes, etc, to get these very fine details, these circuits take time. I think the CSP3 is just coming into itself finer self. All the changes I make are more conclusive, and it is no longer a question of trading one thing for others. It is doing the CSP things without apparent masking. It is all pretty gorgeous really.
Tubes for now anyway!:
Torii MKIV: Mullard labelled OC2, Westinghouse labelled 50's OB3, 50s RCA 5U4G-ST with top side getters, Tungsol cryoset KT66, Siemens PCC88 tall bottle (or maybe cryo'd Amperex Orange PQ 6922).
CSP3: 50s RCA 5U4G-ST with narrow black plates and bottom side getter, power: cryo'd Tungsrum PCC88, input: early 60s Amperex E188CC
This was all from volume matched tests though, and does not consider gain riding, to me, perhaps the greatest asset of an already excellent gain stage. The ability to adjust the MKIV up and the CSP3 down, or visa versa….decreasing or increasing weight and body, but also, highlighting the traits of one amp/tubes over the other. With tubing and other tuning (or with stock gear actually), this can introduce pretty impressive potentials for beneficial fine adjustment of a wide range of recordings. Too dense, up the MKIV volume and lower the CSP….too lean, reduce Torii volume and up the CSP3.
So…to the original question…Steve says the MKIV is good with or without a CSP3…I think this is totally accurate. I do think the "without" could be more relevant with the more powerful Decware amps, already having a lot of weight and drive. And my explorations to date are implying that the amazing MKIV may fall a bit deeper into the territory of making the Jupiter CSP3 less necessary. Since the MKIV can reveal such a sophisticated balance of detail, dynamics, and musicality…and with a lower-key expression of the things the CSP3 does so well (including being surprisingly tolerant of lesser recordings) does it need enhancement? No, the MKIV is great on its own! But I bet most folks would find the CSP3 a very useful sound tool with the MKIV.
Also, judging from how long the MKIV took to eek out those last glorious bits, I am guessing my CSP3 is still coming along….Though I am no longer on the fence, I look forward to the probability of more exquisite sound. At this stage of the game for me, the last bits are the most important. It is a real pleasure and gift to have gear available that allows this!