Harley, the guy at Pass Labs that I talked to, said that the 150.5 has much better sound than the older X-150. I have looked for a used 150.5 but haven't been able to find one. Supposedly the 0.5 series have a slightly warmer sound and are more tube like.

You are right about specs. My A21 amp has higher current, slew rate and a dampening factor over 1000 along with lower THD than the X250. On paper it looks great. But in the real world it can't compete with amps like the Pass.