I am not sure if you would consider something that is not 'pre-packaged' but I got a great sub for just under $600. It uses an NHT 12" woofer, 200 watt mono amp and a oak veneer MDF cabinet. The kits are available from www.madisound.com. You can get the cabinets, choice of drivers, foam, spikes, etc at the website. Overall it is a pretty good sub especially when compared to stuff in that price range. It is much better than the Paradigm sub for sound and looks.
BEST MUSICAL SUBWOOFER, UNDER 1K
I am interested in hearing suggestions about a musical subwoofer, it does not have to be gigantic, but it needs to be musical, fast, so it can integrate well with most speakers I am likely to match them with. Any suggestions from Energy? Paradigm? or others? I would like to pay less than $600 Thanks for any suggestions