I've never owned those speakers, but I'm really just skeptical based on Gene's review of them... he seems to have pumped plenty of power into them without running into distortion type issues (IE the part where he used his monster denon receiver because the 40wpc tube wasn't cutting it in his huge 6000cu ft theater room, it sure didn't sound like the speakers don't love power).
The USP-1 certainly isn't something I'd think of as clipping the pre-outs. That's more or less something I'd reserve for entry level receivers with pre-outs just dumped in for a few extra sales.
On an unrelated side note, I wonder how easy it would be to do a tweeter swap on this speaker with the original 41-SE Vifa tweeter,the same review seemed to suggest that the crossover used is identical.