Well I've been able to answer one question about my subs. So starting with one sub on and then turning a second, then the third and finally the fourth; I measured the output using some pink noise, having all four gives me 11dB more output over a single. So for example one up and running at 85dB, all four up and running was 96dB.
JTR Noesis 228HT (LCR)
Axiom Audio QS8 surrounds
Sherbourn PA 7-350
Falcon Screens FVHD105
Dual PSA XS30's (gone but not forgotten)