I have a Denon PMA-900V that I bought in 1987 along with Polk Audio SDA-2As. At the time, I thought I was making a good decision. However, the Denon is rated to be used for speakers that are rated between 6 and 10 Ohms (I believe) and the Polks are at 4 ohms. I cannot run the Denon at an elevated volume for a time without it REALLY heating up. Is there a way to raise the impedance between the Amp and the Speakers? Would this improve the sound also?