Two Subs?

I don't think you need 2 subs unless your mains are extremely challenged...summing the left and right signals in the "high level" input of a REL and setting it at around 55 hz or so (or wherever your mains start to drop off) is all the bass you might need, but, of course, this is entirely room dependent. Multiple subs can get more standing waves and/or phase issues ramped up, and I prefer keeping my sub away from the gear. I am anti room correction device due to my inflated Professional Sound Mixer ego, and can (and do) adjust my REL level if it's out of balance. I think room correction gizmos are there for those who don't seem to be able to turn a sub up or down, insist on listening at extreme levels, the listening room is in an underground cavern full of medieval torture devices, bats, and dripping with moss, or simply feel so insecure about system set-up they require an electronic helping hand.

Two or more subs are mostly for balancing bass levels more smoothly across a room, not so much to get more or better bass at any one particular spot, though it may well help achieve properly balanced bass levels more easily there as well.

Duke from Audiokinesis is a proponent of using more bass modules for this purpose of helping to randomize bass modes throughout the room better. Of course, any pair of larger full range speakers do this to some extent, but best choice of placement for bass may not always be the best overall.

AK planetarium systems use 4 separate "bass modules" to help achieve this for example. This makes a lot of sense to me and the AK Planetariums are high on my list of potential high value speaker systems (especially for more problematic rooms acoustically otherwise) I want to hear someday.

As one who lacks PSM ego (tho I've got more than enough of the garden variety sort to offend most folks), I do use room correction. Attendant to that task, I've generated tons of readouts from my RTA that show FR at the listening position. Two subs generally produce notably better "looking" readouts at the listening position than one sub, prior to EQ. After EQ, two EQ'd subs probably produced better results than one EQ'd sub over a wide area - tho results at the listening position were awfully close. I say "probably" because the comparison is really dependent on how you define the "wider area".

I also emphasize "better looking" than "better sounding" only because the latter is subjective. However, to my ear, the results definitely correspond to the readouts - I much prefer the articulate bass sound of smoother FR at the listening position. IME, smoother response comes with multiple subs and/or room correction.

Just my own experience, FWIW.

Marty

PS - re: Bob's comment on the high level REL x-over scheme. There are advantages to inserting an active x-over that you forego when using REL's scheme. Believe what you will regarding the claimed benefits of REL's scheme, I'm pretty sure that Bob was expressing his own strong preference for actively crossing to the subs ahead of the power amp.

Sub measurements schmeasurements! I get lots of clean bass, because if I didn't I would be unhappy. Unclean bass indeed! Not sure what "smoother" sound means, but whatever I'm getting from my little REL seems smooth...I think it's dialed into maybe 58hz or something so it's actual low end only to maybe 25 hz (I measured it...so sue me!) before it gives up the ghost. I've measured my listening room using digital real time analyzers with various serious stupidly expensive pro cardioid mics, active and passive pro EQs both graphic and parametric notching types, and other stuff to play with my room system, and I prefer none of the above in the system as it seems cleaner without it. I simply turn the REL up or down...a teeny bit...ahhhhh...perfect!