What sub for Audience 52's

I have a newly acquired pair of Dynaudio Audience 52's that will be mated with A Classe CAP-151 and a modified Marantz CD player with Zu speaker cables and interconnects. My room is 10x12 and I listen to music at moderate levels, mostly blues, bluegrass and a little classical. What would be an appropriate sub that I could purchase for $500 or less used.
Take a look at the ACI Force it is 250 more than you are willing to spend, but it is wonderfule. Here is a link

I am using a Hsu VTF-3 Mark II with Dynaudio 70s. It is a great sub in a large room. This is a home theater setup. I actually like my REL Q108E with Von Schweikert VR-1s better. That room is for stereo only and is smaller, similar to yours. The Hsu is a bit of overkill in a small room. Both are in your price range used. Good luck!
Thank you both for your help.