Hey Tom. I am in the gain riding school, "riding the gain" between pre and amp to alter tone/weight, improving sound based on recording qualities. This is about using the "best" of the CSP3 sound influence range for my system, and letting the amp take care of major volume changes. Here the most beautiful range of the CSP3 (or previously the Zstage) is a fairly narrow area, fine tuned to get really optimal contribution to the overall sound from it within the sweetest gain riding area.

With the tubes I am using now, I have the CSP input tube volume set at 8, the output tubes at 8, and the volume pot I "ride" for fine tuning from about 12 o'clock to 2:30. At 12, the CSP3 is really sounding great, adding its musical refinements, but with a leaner influence. At 2:30, it still sounds great, but adds density and more of the mysterious OTL influence. It is the density I listen for mostly as I ride the gain to get the most from the recording. As a general rule, if the bass is heavy, or the recording is too dense/warm, I will run leaner. If the recording is lean/thin, I will run more CSP3 to thicken/richen the tone.

My Torii MKIV as more neutral across volumes. Unlike the CSP3, where the more you push it, the more the CSP OTL effect and weight is revealed, generally the Torii is more uniform. I will run the Torii from less than 9 o'clock up to more than 3 o'clock and if 9 is very low volume, low enough that you can hear the speakers craving power, I can run up the CSP a little extra, rounding out and empowering the presentation.

Whether you prefer the old school approach - turning up the amp all the way and use the pre as volume, or "riding," with Steve's pre designs there is an element of gain effect. As the pre becomes more of the sound in the balance, the amp becomes less, and in the case of the Zstage and CSP, the more voltage you output from them, the denser the tone becomes along with other tonal qualities the pre is designed for and tuned for. The latter is a tool I like to use also, generally tuning the pre to be a little more open, allowing for more "ride" tuning scope, but also for more tonal density in general without getting too thick.

So with my gear and room, and my preferences, I get a more refined and flexible sound from optimizing the CSP3. This is not maxing it out per se, but I do find the area that utilizes its beautiful character fully within the needs of this room and system, yet before it begins to sound a little unnatural from too much of that good thing.

I agree that whatever sounds best is what you should do.  Experimentation is the key.  I use a Cary SLP 98P preamp with my ZMA.  The preamp has dual channel balance controls along with master volume, so I actually have to find the right combination with those as well as with the ZMA.  

Part of my "needs" are having the ability to allow enough swing in the preamp's volume control for fine overall control.  Just recently I notched the ZMA's volume higher even though this made my preamp volume more sensitive.  I found allowing the beauty of the ZMA to contribute more sounded fuller with better lower frequency body.  I was missing out on the best sound for no reason.  

I still don't have the ZMA full gain.  That would really limit my preamp control too much.  Again, experiment.

Scott