Welcome to the forums, wow where to start.
I guess first of all, did you "re-calibrate" everything once you hooked up the new speakers. In other words, did you run the built in test pink noise and calibrate your speakers using an SPL meter, or the built in mic from the primary seat, assuming it came with one?
I think most likely your room layout could be part of the problem. But others have used a "corner" location without issues. Have you tried moving the m60's a little closer to the wall? This would help reinforce the bass frequencies, but not sure if that is what your having issue with. 12-15" should be fine, but might work better closer.
Are the speakers "aimed" straight out parrallel to your ears, or are they slightly towed in aiming behind you when seated? How far away from the speakers are you sitting? You want to be at least the same distance back as they are apart, or more...
Are all the drivers working? You didn't remove the gold connection between the speaker terminals did you on the back, that needs to stay in place with all terminals snug.
What mode on your receiver are you using when listening to music?
I would think the 60's should blow away those Polks, I know I thought my old 60's were better than B&W 703's which cost twice as much....Now I own m80's...