The Polks are very hard loads to drive for any amp, especially in larger rooms - the LSi15 is probably the toughest, but the 9's have proven to be challenging as well. What you are describing doesn't match real well with what I remember hearing from other Polk owners, though. It is possible that you are hearing flaws in the source recording. Do you recall listening at these same levels in the old room when you were running 3.1? You could do a test: find a passage that produces this break-up, set the 970 to stereo (it would then downmix the surround), and see if you get the same result.