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.
TIA
DG