One sub or two?

Agree with Loydc comments. I used a single REL for few years & then upgraded to two RELs. The improvements were very evident. Imaging improved the most. Although they say deep bass is non-directional this is something you will immediately disagree with once you hear 2 subs.

Crossing over at 40hz the bass wave is so large it is impossible to distinguish its location...however one has to have true full range output to do this...and a speaker either images well.or it doesn't...regardless of a sub

Phasecorrect...

Two subs in almost ANY room will sound better.

what do you connect the subs to? to the L/R speaker? or to preamp?

I actually have a Rel Storm III working together with my B&W 804S, in a 3700 cu ft room. I like it with the Rel a lot better than without, BUT I'm looking at getting rid of the Storm and bringing in 2 subs + Anti-Mode 8033. Rel sounds nice but not very articulated, and I'm crossing it over with the ABC network at about 45 Hz. I'm inclined for sealed enclosures now, and also considering servo-driven subs (Rythmik).

FWIW, in Sound Reproduction Floyd Toole recommends two subs playing the same summed up signal from L & R, placed at 25% of room width at each side of the front wall. So if the wall is 16 feet wide, one sub at 4 ft from one side wall, and the other at 4 ft from the other. By doing this the first 3 width room modes are kept in check. Then I further plan to improve things with DSP by the 8033.

Also FWIW, when I grew dissatisfied with the Storm I was thinking a larger, huge amp-driven sub to replace it. Some folks made me rethink this.

Anyway, my two cents.