Sub. selection

Any thoughts from the collective wisdom as to whether the Def. Tech. Supercube II, the Von Schweikert VR S/1, or any other small sub. would match up well with my Spendor S3/5 speakers? Although one could say the room is really too large for these small speakers and my 45 watt amp (VTL Tiny Triodes), the combo. seems to work pretty well at normal listening volumes. I am only looking to fill in the bottom some, as the Spendors are down 6 db at 70 hz, and not to shake the house. I am limited to a small sub footprint and am willing to accept what goes with that. Music only listening, no home theater considerations. Thanks in advance for any help.