looking for HT sub to match well with current syst

I amlooking for advice on a sub to match my current HT system:
Marantz SR7500 reciever
PSB 6T (L/R)
PSB 9C (center)
PSB 10S (rears)
Current sub Velodyne ULD 15- at normal listening volumes- doesn't have the punch; however, as the volume increase wow does it have punch.

I amlooking to find a sub that will provide more punch at lower volume levels than my existing Velodyne. Suggestions welcomed
For HT HSU Research makes a great value sub, maybe not the most musical though.
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