David I have had both in the showroom and I like the X 600's better. It is subtle and I cannot describe why I like the X 600's better but I do.
Pass is releasing 2 new amps in the same chassis as the X 600 and X 1000 but with an oval faceplate the XA 150 and the XA 200 both class A and they are a mix of everything that the X do and what the Aleph did. Projected price is the XA 150 will be $18,000 and the XA 200 $28,000.
I have trouble imagining that they could be any better than the X 600 my guess is just a touch different.