There are a few Pass Lab owners around here, so FYI. In the latest Stereophile, John Atkinson himself reviews the XA60.5 monoblocks and declares, "I don't have much to say about the Pass Labs XA60.5 other than this: It is the best-sounding amplifier I have ever used."
Solid State Power Amp Reviews | Stereophile.com
The good news is that all X and X.5 series amps have a lot in common.
The bad news is that a pair of XA60.5's are exactly twice the price of a X150.5.