I've often wondered about this, for different reasons like the allure of tubes etc.
I'm run the Pass Labs XP-22, XP-17 and the XA60.6s with an analog front-end only. I recently put in a distributed bass array and altogether I'm blown away and quite happy. Yet...
Curiousity and interest got the best of me and I reached out to Pass. Here was their recommendation for tube pres: Atma-sphere, BAT, Conrad-Johnson.
I'd like to know what the OP decided to do. And any other continuing thougths.