I got my XPA-2 about a week ago and although I didnt believe there would be a 'big' difference between an Onkyo 875 (bi-amped front channels), the difference is damn big.
I've got pretty revealing speakers and and a superb DAC though so the difference on some other systems may not be as large.
Basic differences:
A lot more air to the sound, tons of it.
Lots more detail that previously couldnt be heard.
It controls the speakers better and responds much deeper.
The sound is so compelling, on its first day I did around 12 hours of listening before I fell asleep and continued after I woke up
There is so much extra detail to be had!