I have a Pass Labs X2.5. Excellent sonic performance via the XLR connections. I've not tried the single-ended. Though, I think the user interface is pretty crude.
I have the exact combination (Pass Labs X2.5 and PS Audio HCA-2)you have been questioning about, which is now powering a pair of ML Aerius i's. For about $2,500 you can get a used X2.5 which will reap better sonic improvements over the PCA-2. The difference is not night and day but the X2.5 has a lower noise floor, quieter background, and so you simply hear more of the music. I agree with Stehno, the X2.5 is truly mercurial with XLR connections. You have to take advantage of the fully balanced design (both X2.5 and HCA-2 fall into this category). The PCA-2 is no slouch and is fantastic value with a performance that is actually very close to the X2.5. The synergy between the HCA-2 and the PCA-2 is very very good. I have heard this combination and really liked it. At that point though, I already have the X2.5 and did do some A/B comparisons. Either pre-amp will go very well with the HCA-2, especially via Martin Logans.