I believe Flint has mentioned this before, but one problem for the average home theatre buff. How do you calibrate, 4 subs in different locations? Plus that might give your Audyessy room correction software a fit.
But then again MattB says the more Subs the Merrier!
I'm happy to share how I went about it - but because my set-up is atypical (which I suppose most multiple sub set-ups are) you should probably first read up on my system description in that other section of the Annex.
First off placement was a combination of calculation, trial and error, and convenience. The front two Velos were placed symmetrically on the front wall at distances that broke the span of the front wall into segments of distances that corresponded to prime numbers. The theory being that you would not have as great a chance setting up standing waves / reflections. (Plus as I've mentionned before, the room is not sealed and the whole backend allows sound, especially bass, to escape to the rest of the house - which acts like a big bass trap.) The other two subs are stacked in a rear corner (that's the "convenient" location.) (The fifth sub's in another corner, put as my other thread mentions, its erves a completely different purpose than these four.)
Actually the two Velos and the B&W are wired to behave like a single sub. What I did was to calibrate each one's level alone to match the rest of the system. Then with them all on together (treating them as one) they were again calibrated to the other speakers (which involves dialing back the signal that's sent to them.) A further step (and this is iterative) is to use a Velo ICBM-1's display function to, in real time, adjust / fine tune the Velo's crossover frequency, and all three levels, to yield the smoothest response - which in my case turned out to give essentially a flat line from 15-100 HZ. (From published reviews / measurements I had learned that, with the Velos, changing the crossover frequency has more of an effect that just changing that frequency. The whole curve changes somewhat - and selecting the highest frequency for example does not yield the deepest potential bass response. The ICBM-1 shows how much.) I'd had the three dialed in pretty well by ear but the ICBM-1 really helped with the last step. So those three run in unison - handling all bass-managed frequencies below 40Hz from the mains.
The PB-13 which handles LFE only was then a simple calibration against the rest of the system (mains and three subs - which are now treated as if they were single speakers). I obtained "correction" information for the RS analogue meter which ended up causing me to bump up the signal to it by a few dB.
If someone were to walk into the room and chnage every setting randomly, and assuming I did had not written values etc. down, I could probably set the whole thing back up again in less than an hour. Given the number of combinations and permutations possible it would almost certainly not be exactly the same. But darn close.
I would imagine that doing the calibration for four identical subs in the set-up that the S&V article used would be a piece of cake in comparison. Assume that all you have is level control for each. (If more control than that just set them all to the same - whatever that might be - and by "same" I mean whatever worked best for a single sub.) Once location is picked calibrate the level of each one individually against the mains. Then run them all together again as "one" and calibrate against the mains.
As for the use of Audyssey, maybe that's why I found I did not like what it did and why I don't use it. My guess is that the multiple subs do give it problems. Although it's really not in the bass range that I find fault. But that's a whole different topic.