I use two REL Strata II subs ... more even bass response to be sure, but I can't overstate how much more effort and fiddling it took me to get two subs sounding 'right' vs. just dialing in a single sub. In my room, the secret ended up being to have one of them out-of-phase, and the other one in-phase (and it had to be the correct one, not either/or). That helped me finally narrow in on the elusive 'sweet spot', but it really took me a lot of trial-and-error effort to get there.
With two subs, neither one of them barely breaks a sweat, and you don't even know they're on until you turn them off (if that makes sense :-) and the whole 'rightness' of what you've achieved collapses.
And this probably goes without saying, but definitely run those RELs via the high-level (Neutrik) connections (i.e. off your amp's speaker outputs). Just my $.02.
With two subs, neither one of them barely breaks a sweat, and you don't even know they're on until you turn them off (if that makes sense :-) and the whole 'rightness' of what you've achieved collapses.
And this probably goes without saying, but definitely run those RELs via the high-level (Neutrik) connections (i.e. off your amp's speaker outputs). Just my $.02.