Best Subwoofer for B&W 805s Speakers

Thanks, I'll keep the REL in mind, I know they are very musical since this is at the top end of my budget I was wondering if anyone has had any experience/good results with something a even less costly?

The amp I have is the Rotel RB 1080 and Rotel RCD 1072 CD player, are these components not considered upstream. Also I have bi-wired the speakers. The Velodyne DD series would be my ultimate choice but a bit too pricey for me. I am seriously considering a AV123 MFW 15 now.

I never knew I needed a crossover, I guess that shows my inexperience. The SVS SB12 plus is one of the first subs I looked at, the word on the street I got was SVS was better for HT than music, the HSU(VTF 2.3 or 3.3) was better for music. How do I know whether I need a crossover with a sub for future reference? I already tried out the Martin Logan Dynamo and it didn't sound too bad. I'd like to thanks everyone for their input.

Thanks for the explanation Bob. You have me leaning towards the SB12.

Anyone have any feedback on the Robert Acoustics Vigor? They have B-stocks on sale for $549.