Thanks for the feedback Dennis.
I brought my portable XM unit into the living room and A/B'd the portable VS. the Polk and found no substantial difference in sound quality.
I'm coming to the conclusion that the Axioms and XM are not a good combination for my ears.
I have never, ever considered my hearing to be very acute when it comes to listening to music reproduction, but I hear almost every vocal and much of the other upper midrange material as having a metallic, shimmery sound to the point of being absolutely unlistenable...yet my wife doesn't hear anything wrong whatsoever.
I think I need to ditch XM, my Axioms, or my wife.
