David was most interested in concert halls, and in such large spaces even long bass wavelengths have space to propagate and be reflected from large surfaces. The delays generally responsible for impressions of envelopment are of the order of 80 ms and more. Small rooms are in a different category entirely. In the 3rd edition of my book, the topic of "stereo bass" is discussed, as this is the closest one can get to replicating something that might resemble events in a concert hall. David was among others who promoted the idea that stereo bass was a requirement, and at Harman (where he worked, as I did) a demonstration was set up to test the notion. I titled the section in my book "Stereo Bass: Little Ado about Even Less" with apologies to Shakespeare. That about sums it up, because substantial effort, including generating special signals to reveal differences showed that any audible spatial differences attributable to stereo bass occurs at frequencies above about 80 Hz. Others have found similar results - so bass managed subwoofers sacrifice nothing spatial, but multiple subs are huge advantages to the timbral quality of bass when room modes are tamed. Todd Welti wrote a paper summarizing the topic: Welti, T. (2004), “Subjective comparison of single channel versus two channel subwoofer reproduction”, 117thConvention, Audio Eng. Soc., Preprint 6322.