Tom seems to have covered the technical differences, and in a much better way that I could have. Another difference you may be concerned about it how efficient these subs are. The EP350 has a 200W amp, but it draws nearly 500W from the wall at max output (so its about 50% efficient). The EP500 on the other hand is about 90% efficient, so it should only draw about 550W from the wall at max output. The EP500 also has a dynamic power supply, so when little/no sound is coming out, there is very little power draw from the wall. Since I'm disabling the stand-by on one or the other, this is an issue for me since the EP350 will draw a lot more power then the EP500 when its sitting idle.
As far as sound goes, the EP350 works excellent with the rest of my setup. I haven't heard many fancy subs, but this is definately supperior to anything else I've heard. I just watched Star Wars II a couple days ago, and
. It was shaking everything in my room with no signs of distortion at all. For music, its very accurate and clean. If you've ever heard "Dragonforce", their uber-fast double bass drum was still separated (if you want to hear a little from them, stop by www.pandora.com
and put "dragonforce" into the search). It may have been blurring them together just a little bit, but this was amazingly small compared to my old crappy sub which just tied them all together into one solid rumble.
As for response, it seems to emphasize the lower bass to some degree, but its most likely due to my small and oddly shaped room. When I tested it, I found that it drops off fairly quickly below 28Hz. I didn't create an FR curve, but it sounds fairly consistent from ~28 up to and above the crossover. There is no sign that the crossover even occurs, it blends seamlessly.
As for stereo between the M22's and EP350, its works just as good as for surround music and movies. It sounds very realistic and accurate.
My reasons for upgrading:
1) Less heat/power with the stand-by disabled in the EP500 then with the EP350.
2) EP500 allows me to avoid upgraditis for a long time, until I break it.
3) I can't break it since the DSP limits the max output to the driver.
4) I don't know what size of room it will end up in when I move out, but it should fill it with bass no problem.
5) Those room adjustments would be nice to have.
6) It plays quite a bit lower and louder then the EP350
7) Every review and forum member who's commented on the two say that there is a huge improvement between the two. Its been explained in many ways, so I'd suggest reading some reviews on it rather then me trying to relay what they said.