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.

Two great pieces of equipment to decide on and may be a tough choice.

I have pretty much the same Decware gear as you (plus the 2 other pieces you're considering).
I also have the CXNv2 with MWI mods like you as well. I'll put my 2cents in here and I hope it gives you more information to help make a decision.

The CXNv2 with Modwright mods is fantastic and a great digital frontend. I've had it for 2 years now. The preamp feature is a nice option but will not give you adjustment capabilities like the CSP325 or the CSP3 w/A mods. I was hoping to use the volume control in preamp mode (for remote control) and although it works fine, I find it takes some magic away from the mods sound quality wise, so I don't use it.

Early on I was experimenting with the XLR and RCA outputs and really couldn't detect any volume difference. I was getting great sound but didn't notice any more heft/density with one or the other. I expected to have to change volume settings for A/B testing, but I was getting the same output with either set of outputs. I wasn't sure why, so I decided to contact Dan Wright and ask.  Here's what he said:

"The RCA and XLR's output the same signal level because they are both transformer coupled and in parallel. The output is at least 4Vrms for both RCA and XLR outs."

Once I heard that it made perfect sense. Exactly what I was experiencing. The sound was great, full, and rich no matter which output connectors I used.
The really interesting thing about this is that I have decided to use the XLR's with the ZBIT but to actually turn DOWN the voltage feeding the ZROCK2 (w/ full A-mods). In my case, the ZROCK2 likes a little less voltage before it goes to the CSP3 (A-mods). The synergy is better than just connecting the RCA's with higher voltage. I can get a buzz from the ZROCK2 if pushed too hard. (at least in my system)

At this point, feeding the CSP3 with a nice clean signal from the ZROCK2 allows me to then dial in the master volume control along with the 2 input controls and 2 output controls. Add the advantage of tube rolling in the CSP3 and it's incredible. The CSP3 also has two RCA input jacks, and I can plug in a phono preamp to it as well. The CXNv2 doesn't have RCA in's but I do use a CD transport into its digital in and take advantage of the DAC. Really nice.

Then, passing that signal to the UFO25, it is then transformed into indescribable music coming from the speakers (in a disappearing act) but I don't have to tell you that I'm sure! Add the gain riding and tube rolling with the UFO25 and the possibilities are endless. In fact, at first I thought I had bitten off more than I could chew buying this complete chain with it's almost infinite possibilities of adjustments and gain riding, but I'm learning how to control it and having a blast.

To be clear I have tried all possible combinations of the chain. I first started with just CXN to UFO25 (w/ RCA's) directly. Sounded good. Then CXN (w/ XLR's) to ZBIT to UFO25. A little better since I had more control over the gain of both units. I won't bore you with all the combo's but suffice it to say I've tried it all with and without individual components in every configuration. Bottom line is I always come back to the complete chain (CXN - ZBIT - ZROCK2 - CSP3 - UFO25). They all bring something special, and they make it a complete system to my ears.

The only thing I can't do is place the ZROCK2 between the CSP3 and UFO25. If I do, I get a buzz through the speakers that is not acceptable. Your system synergy may behave differently, and you might find it works perfectly fine. I've tried a few times but to no avail. Not a big deal since it works great exactly where I have it.

Lon and Will have given some great advice and something to really consider as well.

Since the voltage out of the CXNv2 isn't any different from the RCA's or XLR's (so seemingly no different in heft/density/etc.) and you're happy with the RCA's directly to the ZROCK2 then maybe you don't need the ZBIT. However, if you think you can benefit from adjusting the voltage to the ZROCK2 then it might be perfect (adding a good set of XLR's isn't trivial though).

The CSP325 (or CSP3 with full A-mods which keeps the headphone jacks if needed) is a great addition because of its gain riding ability and its own special ability with tube rolling options to completely tailor the sound. This piece of gear is hard not to recommend.

I'll propose something here. Make this a win/win situation for both you and your children. Get both!
You'll get to thoroughly enjoy this gear and when the time comes, they can sell it probably for what you bought it for and recapture the funds you were hoping to leave. In the meantime, you get to enjoy what you deserve and hopefully share it with your children as well.

Anyway, decisions, decisions! Good luck!