If you have the Denon x 3300, then it has Audyssey Sub EQ HT, which has the ability to EQ two differnet subs..
You have not mentioned your budget.. That should help narrow down models..
Most recommendations out there for sub, insist at-least dual subs, which helps bass across the room.. So whatever budget you have, try to accommodate dual subs (identical) and not chase a specific expensive brand..
Even if you have a single expensive sub in a corner, it might not give the best results in your listening position..