Julian,
Ignore impedance ratings; "impedance" is an electrical trait of speakers and has nothing to do with sound quality. It tells you only if a speaker will draw more current from an amplifier at some frequencies (impedance varies with different frequencies) and therefore be a more difficult load for some amplifiers/receivers that might be unstable driving 4-ohm loads. Larger multi-driver speakers like the M80s tend to have lower impedances in the region of 4 ohms, so we recommend some brands of amplifiers and A/V receivers that have been proven to be stable driving 4-ohm loads.
Higher-impedance rated speakers (6 ohms, 8 ohms) are an "easy" load for any receiver or amplifier; for example, the M22s are rated at 8 ohms and are easy to drive.
As to center channel speakers, the VP180 is a no-compromise center speaker that is spectacularly good for any type of music or dialogue. I'd call the VP150 "pretty good"- but not in the same class as the VP180. Note that the VP150 is perfectly satisfying for movies and home theater. It's just that when it comes to nuances of, say, male and female vocals, the VP150 isn't as smooth and totally free of flaws as the VP180. But understand that I'm a bitchy critic of loudspeakers and sound quality--one of the reasons Ian hired me--and in the double-blind listening tests, the VP180 was the first center-channel speaker I found completely free of any sonic flaws. It's essentially an M80 on its side with the drivers rearranged and the crossover modified significantly.
Alan
