The amp has very little to do with everything is the install is sub par and you using low quality mp3's.
I used the RF R series amps already, they shouldn't even be on that list with those other two amps.
Anyhew, there is a Polk amp for sale on tuner at a reasonable price.
viewtopic.php?f=8&t=627667&hilit=polkand ted have a DD 4 ch reasonable.
I'd look into them.