Need some Amp help - a little new to properly powering speakers

What Rob and others said. Although, I guess he was joking about the Quads. Your rig sounds pretty good at high volumes but giving it bigger amps will only make it a little louder. You need to get better quality gear to make it sound better at lower volumes.

Agree with the sentiment of pursuing higher "quality" amplifier (true for all audio products make quality the priority ) rather than higher power. 

i agree with the foregoing sentiments (which makes me re-think why i'm posting...). more watts, as opposed to higher-quality watts, won't really help. your yamaha seems like a good match for your speakers; you seem to enjoy it and i'd be inclined to leave it as is.

I don't know of any proper test of the Yamaha, but it may well be pretty good (and not mid-fi) if it compares to e.g. the Yamaha AS700 tested here: http://i.nextmedia.com.au/Assets/Yamaha_AS-700_Amplifier_Review_LoRes.pdf
The need for power largely depends on the music you play, the size of the room and the sensitivity of the speakers (88dB in this case). Unless the room is very large 150 watt per channel should be OK, even if (up to a point) more is always better (for cleaner response to dynamic peaks). More amplifier power does not need to cost much, of course. Better speakers would make a bigger difference, but will cost significantly more.

FWIW, Polk in general are looked down upon by audiophiles with some good reasoning. Vintage Polk not so much as they were much more suited for the audiophile world. Polk headed in a downward spiral after they courted the mass market and nothing they produced after could match their vintage products. I am currently, and have been for years, running an HT system with all vintage Polk speakers powered by an Emotiva XPA5 and a Lexicon processor.  That said Polk speakers require some power to make them sing but 250w is more than enough to get there with plenty of headroom to spare. It' kind of ironic that you should be driving the speakers with a Yamaha as I experienced similar issues when I had mine mated with a Yamaha processor. The yammy had to go. I also strongly suggest fixing your listening environment before spending a lot of money on equipment. Nothing you buy will sound "right" until the supporting environment is corrected. Doing that will save big future dollars. You might also find it solved your immediate problem. Just saying.  PS: Emotiva does make exellent and affordable equipment in its price range. It will never compete with the higher end equipment and it is not meant to.