I second the CJ suggestion if you want to give tubes a try. I have Response 2.5's and use Premier 12's with it and I think it sounds fabulous. They're also known to pair well with ARC amps (either VT100 or 100.2), and I've heard them sound very good with Cary. A used CJ Premier 11a (~50w into the ProAc's) will run between $1500 and $2000, and Premier 12's (~100w) between $3500-$4000. Keep in mind that power isn't necessarily the most important factor as long as the amp is capable of driving the speakers. Tube equipment tends to clip more softly than SS, and so can be driven a bit harder than their SS counterparts and still sound good.
One caveat is that the ProAc's have a pronounced bottom end which can end up being a bit overpowering in a smallish room with a warmer amp (which is what I experience in a 15' x 20' room). However, these speakers simply sing when driven by tubes and I'd give serious thought towards leaning in this direction.
One caveat is that the ProAc's have a pronounced bottom end which can end up being a bit overpowering in a smallish room with a warmer amp (which is what I experience in a 15' x 20' room). However, these speakers simply sing when driven by tubes and I'd give serious thought towards leaning in this direction.