I need a musical sub under 500,,,,new or used.

Paradigm PS-1200 Subwoofer. This is a really good sub for music and movies. Alot of reviews on Audioreview about it.

Believe it or not, about ten years ago Pioneer made a subwoofer called the "SW-1000" that is remarkably fast and musical.... I have one in my second system and while it's certainly not as quick and won't go as deep as my REL Strata II, or play as loud, it performs a lot better than you might imagine. The Pioneer has speaker-level inputs which are very transparent-sounding and 12" paper cone woofer that is quick and tuneful. It is without doubt more musical than bandpass-type subs (like the Paradigm PS1200) and IMO more musical than the budget Velodyne, Klipsch, JBL and other low-end (10" woofer) subs. I've seen them go for about $100 on ebay which is a stone-cold bargain considering the performance you can get from this 20" square black box. As an added extra the Pioneer is quite attractive looking with a piano-black top and front-mounted controls. Oh and BTW, I use mine with an ASL Wave 8 tube monoblocks and Triangle titus speakers and it holds its own with this budget audiophile gear.

Adire Audio - rava $399. Sealed 12" Shiva based sub. I am real impressed with mine.

Technically, the Hsu is as good as it gets at this price point. That doesn't mean it'll sound best to you, in your room, with your mains. But it is worth an audition. (Try to find somebody who has one; otherwise, you've got to order one on the 30-day plan.)