I used to own the XA30.5; it was a great amplifier. It replaced my Decware Torii Mk IV, which I was sure, was going to be my ‘terminal amp.’ When I got the XA30.5, I thought I was taking a huge chance on it. I didn’t expect it to perform well against my valve amp and figured there was a 50/50 chance I would be disappointed and sell it. I was surprised to find that the Pass Labs XA30.5 eclipsed my Torii in almost every way.
The XA30.8 sounded different in a couple of ways. For starters, it sounded great cold! Maybe better than the XA30.5 warmed up.
There are plenty of other differences between the two amps. For example, the 30.8 is darker, much smoother, clearer, images better, and voices may be pulled back just a hair. It's got more mid-bass, which I like a lot (reminds me of my tube amp). Its bass is overall much punchier, tighter, clearer. The 30.8 sounds much more powerful overall.
On top, the main difference between the XA30.5 and XA30.8 is that the XA30.5 has a 'sweetness' on the very highest frequencies. I don't hear it with the XA30.8. I liked the sweetness on the XA30.5, but there are bound to be a couple of gives and takes.
I can see why you like the 30.5!!