I think that you're right, you're going to have to try the XA.8 in your own system.
I think that the reviews I've read and my experience say that the .8 amps are a little more revealing and have better bass. The .5 amps are a little warmer. But I couldn't guess how that would work out in your system. You may just be making a sideways move.
If you're really itching for an upgrade, call Reno HiFi and get a .8 to try out. If you're basically happy with your amp, save the money and enjoy what you have.