8" and 10" can be better for music IMO, however when push comes to shove and you want the output, a single 8 isn't going to deliver. There are some very good 10" subs out there that can, including those from HSU. SVS would definitely be another to consider with their PB-10, also under $500 (possibly even shipped, depending on where you live). The PB-10 is the least expensive, with the 25-31PCi being a close second by about $50 - everything but the PB-10 are 12" subs from SVS though. They are fine with music as well. Neither is "better" in my book, they both make excellent subs. I have owned subs from both and wasn't disappointed with either of them.
What I might consider if music is a priority, is a sealed sub, possibly something like the SVS SB-12Plus (which is unfortunately $699, not $500), or one of the offerings from AV123 when they start shipping the new subs.