Hi Daveb,
Nice dogs (what's he licking on that table?), nice rug.
I don't specifically know that Polk speaker you're using, but if I were to recklessly generalize--based on years of listening tests to American, Canadian, British and European speakers as an audio magazine editor--Polks are/were wildly variable and inconsistent in sound quality. Some were decent, others very colored and unnatural.
The main difference you'd hear if you replaced the Polks with Axiom M60v3's is more neutral, transparent and natural sounding instruments, with greater soundstage spread and depth (depending on the recording). With M80v3s, you'd get considerably greater bass extension and output and the ability, given sufficient amplifier power, to play cleanly at extremely loud levels. The level of transparency and neutrality of the new M80s vs the M60s would be similar, according to my blind listening comparisons.
By the way, with the speakers pulled out a bit, the fireplace would not affect the imaging or soundstage at all. It's only adjacent surfaces/shelving that will cause interference effects.
The center is a problem. I'll think about it; perhaps others here will have some ideas. An M2 center will work fine for music but obviously it won't have the same loud playback capability as a larger center channel speaker.
Regards,
Alan
