Dual vs single sub

What are considered "bass frequencies"? Below what frequency are the L and R channels summed to mono? One way to hear differences between L and R is to use the Dynaco Quadaptor; it uses the difference (out-of-phase) content in a recording to produce a L minus R signal.

I have been provided conclusive evidence of Duke's (audiokinesis) integrity. Due to my postings on the subject of OB/Dipole subs (particularly the one by GR Research/Rythmik Audio), I was contacted by an Audiogon member concerning their use with planar loudspeakers. I answered his questions, and a few days later received another email from the member, telling me Duke had told him OB/Dipole subs would be a better choice for use with his loudspeakers. How rare is THAT?!