>>What are the differences between the M60 and the M80's? Also, since the M80's are a 4ohm load, would a different receiver be better?

From a quick look at the specs page :

- M80 sensitivity is 2db higher, ie a bit more sound from the same amount of power

- M80 is 4 ohm, ie the same amp will put more power into the speaker at the same volume control setting because the 4 ohm speaker draws higher current for the same voltage. The dark side of this is that if your amp can't DRIVE the higher current then you might overheat or go into current limiting / shutdown

- low bass goes a few hz deeper on the M80 vs. M60

Bottom line is that the M80 is a bit bigger than M60 with all the benefits and drawbacks you would expect -- a bit deeper bass, a bit higher sound output, a bit more effort finding the right placement because of the deeper bass. Being 4 ohm it can also suck more power (= more sound) out of any given amp unless the amp can't handle it.

From a quick skim of HK, Denon and NAD receiver specs (630 vs. 2805 vs. 752, just happened to be what I had on the PC) the main difference between 4 and 8 ohsm seems to be "dynamic power", ie ability to handle the peaks without clipping. Reading between the lines the 4 ohm impedence can be used to get more headroom and avoid clipping but if you suck more power out of the amp on a continuous basis you will still overheat. That doesn't seem like a problem, however, since the extra headroom is what you are looking for IMO.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8