I have the BX8 from M-audio. I am thinking of buying the M-Audio SP-8S Subwoofer. The cheapest I have seen them for is $400, and before I make the plunge, I thought I should ask what is the community's experience with this sub. Also, for the money is there a better buy out there (in the $500 or under range)?

In that price range, it's a tough act to beat. I did recently hear a system configured around the newer SVS sub, it's slightly over the mark you've set, but it sounded pretty darn good. Although it was designed for home theatre, it's pretty well consistent with what you can do with the M-Audio. We tour with the M-Audio, so I can't complain about it at all. We bought it, because we liked it. But...you asked for alternatives. I don't know what SVS service/customer service is like.
http://www.svsubwoofers.com/subs_pb10_isd.htm

Thanks Spot, I'll give a read. No big hurry, but I want to get something in the next few weeks.