I don't know if your application is perfect for a sub. IMO the best use for a sub is the bottom octave of bass that your main speakers won't produce. 70 Hz is pretty high for a sub where your ears can easily locate it. I had some minimonitors that went down to about 55 Hz and no matter what I tried it was difficult to locate the sub so that you couldn't hear it as a separate entity. I now run my Wilsons full-range and just use my REL sub to produce the lowest bass, i.e. to pressurize the room. I find the best way is to roll off the sub with its low-pass filter and connect the high-level sub inputs to the speaker outputs on my amp. Maybe if you locate the sub near the main speakers, or better yet, buy a pair of subs and locate them near your main speakers.
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.