In the past I have heard that the Pass' have incredible midrange but they do not have good woofer and thus bass control.
Rydenfan (Threads | Answers)
All I can say is with my XA-.5 series amps, this is not the case. The XA-.5 and X-.5 should perform equally well in the bass department as they both double power output as speaker impedance is halved.
On the other hand, Bryston amps control bass incredible well regardless of the version.
Pick one...or one of the others mentioned.
The bottom line is you won't know how any of these amps perform in your system until you install one or two.
Buy a used version of whichever amp you select, and your risk is substantially reduced. Bryston, Pass Labs, McIntosh and Ayre all have excellent resale value on the used market.