I’m looking for a birthday present for myself to arrive on my 70th. (Perhaps you can guess my age.) The CSP325 is an obvious choice to pair with my SE84UFO25, but I already have a built-in preamp in my streamer/dac with 2V unbalanced and (probably) 4V balanced outs—a Cambridge CXNv2 with Modwright’s treatment.

Another option (wrapped festively, of course) would be to add a ZBIT instead (saving money and allowing me to pass on a few more bucks to the kids). But would an XLR cable from the streamer/dac to the ZBIT (then on to my ZROCK2 before the amp) offer the same gain riding opportunity as a CSP325? Might the ZBIT be a reasonable compromise?

If you are running 2V now, and the ZBIT allows 4, you would really notice the extra gain I think. But it is not adding gain itself, or much for signal colorations...more acting as a potentiometer on the 4 volts balanced out of your streamer. Most don't run the ZBIT wide open, but will get a sound improvement anyway, the clarifying influences of the transformers and from more voltage increasing density, clarity, hit.

With your ZRock, I think you would have a nice combination... the Zrock having some gain, coloring the sound lighter or darker, as well as adjusting highs down if you like that mode. Together they are nice here, gain riding them with the amp having quite nice tuning potential.

The CSP3 is a whole different thing... OTL, lots of tube tuning choices, loads of gain potential, doing the voltage things to the signal with more flavors that have more potential for personalizing.

I don't think you can go much wrong either way.

I use all three most of the time, and do not use any for volume. I use them for adjusting signal qualities, each imparting their own flavors that can be adjusted for system/room changes and for individual recordings. My amps are generally more my volume controls, those being more neutral across the power range.