Thanks for the thoughts, sirquack!

Just for clarity, my receiver is an Onkyo TX-SR504. 7.1 receiver, listed as 75 watts per channel.

I didn't do anything to "re-calibrate" after changing speakers, and that may be a fair point. I don't have an SPL meter and I don't think my receiver came with a setup mic (again, it's a fairly low-end receiver), but I will check the receiver's owner manual and see what - if anything - they suggest for new speaker setup. There is likely something.

I have tried the M60's a little closer to the wall, but it didn't really seem to have an effect on the sound to my ears. The bass from the M60's is really quite satisfactory, it's more mid-range and higher frequencies where I feel something isn't right. They just don't sound "airy" or "spacious", if that makes any sense at all. For the sake of being thorough, I've also played them with the Polk sub both on and off.

I do have the speakers pivoted *just a wee bit* in toward the center. I measure the distance between the inner walls of the two speakers to be about 6.5 feet at their current placement (the TV stand, of course, sits in the space between); my primary seated position is about 9 feet from the left speaker, about 11 from the right (it occurs to me I did not input the speaker distance values in the receiver). So I think the distances are not particularly off.

I didn't remove the gold "bridge" that connects the pairs of terminals on the back. I am using banana plugs on the ends of my speaker wire. Perhaps I should try bare wire?

I'm listening to music in primarily stereo mode, which engages only the M60's. In tinkering I have tried "Dolby PLII Music" mode (which I didn't care for at all) and "Neo6: Music" mode (which was the far superior of the two, IMHO) just to engage the center speaker also. But after a moment I always put it back on stereo mode.

Another thought: I have the powered sub connected "in-line" with the L and R channels, because that's how the manufacturer recommended to set it up. It's not connected to the receiver's LFE output. I have tried the sub both on and off, but could the sub be doing something that's degrading the signal to the M60's? The sub has a dial to set the crossover freq, but the lowest it goes is 60 Hz (which is where I have it set); but again, the bass seems quite satisfactory to my ears.

I have a several-year-old 100-watt Nakamichi stereo receiver in my bedroom that is only used for playing CD's. This weekend I may take the M60's into the bedroom and connect them to that receiver just to see if they sound different.

Again, thanks for the insights!