Decisions decisions.
I've owned a pair of 500s and a pair of 600s and they are outstanding subwoofers, far better than most subs I have heard especially some hyped ones I'd rather not name.
If you want that extra bit of performance then the 600 is one of the finest subs in the world. The extra performance comes from its larger enclosure, port and amp. Its not just that the 600 runs deeper and louder, the larger vortex port also makes a difference to the texture of the sound, there is virtually no colouring of the bass since port turbulence is non-existent. That is in no way a knock on the 500 since they share the same componentry and design characteristics. The 500 would be a step up in sound in just about any HT. Its just that the 600 is the no compromise version of the 500, a super 500 if you will.
Also with a room that size I would opt for a pair of 600s, or a pair of 500s. Multiple subwoofers really improve sound quality across more seats in an HT by smoothing many of the nulls and peaks. Either way congrats on choosing a spectacular subwoofer.
