You've got some choices to make

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Let's start with your DAC. No surprise it sounds better from the XLR outputs because it is fully balanced. That means it outputs 4 channels: L+, L-, R+ and R-. You get all 4 on the XLRs but only the + half on the RCA. So if you use that RCA output you're really only getting half the DAC you paid for.
If you stick with the CSP3 it means you get the (not optimal) RCA output from your DAC and don't take advantage of the XLR inputs that you spent good money for when you got the 2.1. That seems like one or more steps backward on more than one level.
Maybe the simplest thing (besides just punting the whole preamp idea) would be to change your order from the CSP3 to ZTPRE so you can go balanced from the DAC through the preamp to the amp. I guess another option would be to get a ZBIT to convert the balanced output of the DAC to single end for the CSP3, and then go RCA to the 2.1. But then you just bought your XLR inputs all over again.
FWIW when I was in your position instead of getting a UFO2 with XLRs I bought 2 UFOs and got XLR -> 2 x RCA custom interconnects to make them balanced mono blocks.

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Hope this helps.