Dual sub-woofers??

[etude]

For a subwoofer, "near field" means only one or two feet from your ears -- beyond that point the majority of bass energy is reflected off room surfaces rather than going directly from the subwoofer driver to your ears.

Richard, could you elaborate a little more on this? Surely the characteristics of the room must be taken into acount here. As a general rule, the dominance of the direct and reverberant field in a room is determined by the liveness of the room and the location of speakers and listening position. This is generally applied to a full range source, but I expect that in the bass there would also be a similar relationship. In particular the nearfield region, IMO, would be extended if the walls were designed to absorb bass energy. It should also be impacted by the size of the room.

Time delay should not be a problem for HT, receivers normally have this function.

My experiments in an average sized room, although not extensive, have shown that the bass is subjectively stronger this way than placed in corners at a greater distance. The increase in output for the same input signal and power is quite noticeable. I have not tried this in other rooms yet however.

Tom Nousaine did research presented September 1997 in an AES paper preprint 4558.
He found that room modes don't change much from using multiple subwoofers and
that using multiple subwoofers usually made the bass frequency response worse
than a single corner subwoofer.

I haven't seen that paper so I can't really comment. Where can it be obtained?
My understanding that multiple subwoofers smooths room response is based on comments from many including notably Dr Earl who recommends the use of 2 or 3 subwoofers in a home theatre to smooth the response.

My point was that adding a second subwoofer would lower harmonic distortion (assuming the same SPL as before), but there could only be an audible improvement if the harmonic distortion from one subwoofer was audible to begin with.

Your point was understood. It makes perfect sense that a 2nd sub will reduce distortion for the same output level. My point was simply that the measure of harmonic distortion does not correlate to the perception of distortion. You mentioned that 5% THD was audible. If I understand the study correctly, then this is not true. eg. a music sample containing 0.1% THD is shown to have much higher subjective distortion than the same track with 9.6% THD. This is a significant study as it shows that THD does not relate to the subjective distortion level. One could be convinced to buy one amplifier over another as it has a slightly lower THD. It is the shape of the nonlinearity, not the actual level that the ear perceives most.