Fun! The first thing I would do if you have not already, is not consider the CSP3 as a preamp, but instead as a sound shaper. It has such a wide range of voltage output, you can do a lot to tune your overall sound with just the individual tube pots and the CSP3 gain.

I might suggest starting by setting the front right/left pots at 7, and the back ones at 8. Then, set the Rachel attenuator at whatever level you are used to for a satisfying volume for your room and tastes, and bring the CSP3 attenuator up to where it sounds about as loud as you were used to with just the Rachel. Then try "riding the gains" between the CSP3 and SE34 while keeping close to the same volume, to hear how the CSP3 voltage (and all the rest of it) adjusts the sound, when increasing CSP3 voltage, emboldening your sound, and reducing it in the blend likely opening and leaning the sound. This can be really useful for tuning to different recordings.

Toward your end of bringing up bass some... for example...if you liked your amp at half volume, maybe start setting it there without music, and the CSP3 turned down. Then with music, bring the CSP3 master gain up to fit that volume you liked in your room... Then, to amp up the signal, keeping the volume as close as possible to the same, turn the CSP3 up while riding the Rachel gain down at the same time.

This will give a good sense of how the increased voltage of the CSP3 can embolden the CSP3 and SE34 sound qualities, likely including bass, weight, lucidity, dynamics, space, etc, while introducing the fun of using the CSP3 as a sound refinement tool rather than a great sounding volume device.

I was originally running three 6N1P Decware red tipped tubes in my fully modified CSP2+ and enjoyed the sound.  Since I run a ZRock2 before the CSP, I am able correct the bass output. But I do know what you are saying about the bass being a bit thin when my ZRock is at unity gain.  I would say the bass is not as punchy with less authority.

Anyway…I have recently switched the two rear tubes with some NOS 6SN7 RCA VT smoked glass tubes.  As Will described the CSP as a sound shaper…I would say that these tubes make the bass present itself as warmer, holographic, with more authority.  It’s not slow nor is it syrupy..it gives female vocals a more natural sound. That’s the good news.  Now the bad…those tubes are not cheap and you need converter bases for them.  And to top that off..the tube socket screws prevent the base converters from fully seating….so you may also need socket savers to help the converter bases clear the screws.  Does that make sense?  Hers a pic to give you a better visual.  Hope this helps.  

Thank you to all who replied.  Really appreciate it.  Will, when you say the "power position", are you referring to the input tube on the CSP3?  If so, what were you using for the output tubes?  Thanks.

Hey vinoman. I think of the power position tubes in the CSP3 as the output tubes, the pair between the input and the rectifier... This is where Dom is using 6SN7s in his pics, and I mentioned liking several 6CG7s I have tried.

Did your Rachel without the CSP3 seem a little lean to you? If not, as I pointed to, playing with adjustments and gain balances between the amp and CSP3 can make notable differences. And of course tubes can too, just that I think it would be good to have a good sense of what effects you can get from gain tuning, and getting settings close to optimized between the components before exploring tubes. It will likely make any of your tubes "better!" I am highly dependent on gain tuning for sonic tuning... for refining system balances with tube and other system/room changes, as well as for making different recordings styles play more optimally in my room. Even with good recordings that can sound pretty great without adjustments within my average area for optimized gain settings, I use master gain tuning most of the time for fine tuning, sort of second nature for optimizing recordings.

For years I mostly used variations of Mullard E88CCs with parasol getters in the CSP3 output. They are a pretty warm and powerful sounding tube, with good extension and rich complexity, that is, as long as I used relatively transparent and fast rectifiers and inputs to temper the Mullard's full warmth some. Otherwise these particular tubes were too dark for my tastes...slowing and thickening things too much for me. I often used cleaner and faster tending PCC88s for inputs with them. Some favs were variations of the Mullard CAJames mentioned for this use... here, relatively extended, open, complex, fastish, and slightly warm, but not at all thick or sluggish.

I am not home now, and have not been for a while, so can't say what I have as outputs in the CSP3 now... but I seem to gravitate more to 6CG7s in recent years in that position. And like all of the tubes, you can get a fair bit of sonic variation with these, warmer or more neutral, more or less complex or rigid, faster and a little slower, etc, etc, so it is nice that exploring them can be relatively inexpensive. One that comes to mind for me as particularly warm, but without daunting dark slowing thickness in my setup, are some yellow labelled three plate Westinghouse. On the other side of things, I have some Raytheon labelled, and some Philips, Japan made versions of what I believe might be Mullard designs. Seeming to be made by different factories, these two sound different, the Raytheon labelled ones more neutral to me, and the Philips more bold, but both are quite awake sounding neutral and transparent versions of the 6CG7 type. Characteristic on both versions I have, is a long riser to the round getter. Then, there are a lot in between with the ones I have explored.