> The idea was that by pulling the loudspeakers out far enough, SBIR will manifest at frequencies low enough that they're only occasionally reached by most recorded material. <
Here's another idea: Put the loudspeakers as close to the front wall as possible, raising all the interference frequencies high enough to be cured with relatively thin absorption. As I explained previously, this will give a shelving increase at all low frequencies. But that is much less damaging than a series of numerous peaks and deep nulls. You could also counter that with a simple bass rolloff using a receiver's built-in tone controls.
--Ethan