I am thinking NAD M25 or an Anthem A5 (or a P5 if I can find one on CAM - I missed out recently on a mint condition P5 going for $3500.00) I picked up a Pioneer Elite N-50 network audio player last month and it has made a noticable improvement in the sound quality of my flacs so naturally the next upgrade should be an amp
You'll probably get more bang for your buck by improving the front end than adding amps, and WAY more if you address room acoustics if you haven't already. YPAO is good, but a fair bit behind the curve of Audyssey XT32 & Anthem's ARC and all of them can really benefit from passive treatments first.
That being said, I had the M25 with my previous S4's and it was an outstanding match. I would still have it if I didn't sell it as part of a package with the M15. You should also look at wyred4sound's ICEpower offerings as well. Been using ICE with my Paradigms for eigth years now and they're tough to beat for a combination of power, performance and value. The PVA's were actually my favorite of the Anthem amps, for music anyways, if your room is on the smaller side they're another option.