I don't have a simple answer but here are some thoughts :
1. With M60s, you are getting the sub for movies not for music. I tend to play music with the sub out of laziness (I need to reprogram the bass management on my HK to automatically flip to "sub off" and "mains large" on music), but whenever I am listening for a while I still tend to turn the sub off and listen with just M60s.
2. Regrettably, up to a point you really do get what you pay for with subs. Somewhere between the EP350 and EP500 price range is where things start to level off. It depends a bit on how much of a bass-head you are -- a lot of people are real happy with EP175 / STF2 kind of subwooferage, others aren't happy until the furniture moves across the floor on most DVDs. Any idea where you fit on that continuum ?
3. If you decide that you are a real bass-head the difference between EP350 and EP500 is apparently pretty noticeable. There are some alternatives between the 350 and 500 but not many -- a Hsu VTF3 tuned down to 18 Hz is good as are the bigger SVSes, but both of them will cost you about $1000 up here in Canada. The hot deal right now seems to be the SVS PB-10 but (a) you have to get it shipped up from the US which is a bit of a pain for service (I own an SVS sub
) and (b) most of the reviews seem to indicate that it isn't all that great for music compared to the rest of the SVS line although it seems to be real good for movies.
So... how much of a bass-head do you think you are ?