If you don't want to add an amp (or can't because your receiver doesn't have pre-outs), add a powered subwoofer or two and cross the speakers over higher. That will take an enormous load off the receiver and should help. The sub will also play cleaner and lower than the polk towers.
Edit: also try not bi-amping and see if it makes a difference, you're not getting any extra power to the speakers the receivers bi-amp mode does nothing unless you have external crossovers.
My setup: Sonus Faber Venere 2.5 and center, Ascend HTM200SE surround, NHT atmos minis, dual Rythmik L12's, Pioneer SC-95, Minidsp DDRC-88BM, Rotel RMB1077, Samsung PN60F8500
Last edited by brianmlamb; 06-08-2016 at 10:50 AM.