One Sub or Two?

I went from two REL 510s to a single Carbon Special and in my system it resulted in an upgrade. Perhaps I never had the 510s perfectly dialed in, but how much can one futz with placement and settings before calling it good? The Carbon Special immediately played deeper and faster, and I couldn’t detect any drop in ambience and soundstage by switching to a mono sub. 

One thing for sure is the 510s were significantly faster and more defined than were my previous Rythmik L12s despite the latter having servo controlled woofers. This probably shouldn’t be surprising though as a single 510 retails for >$2500 while the L12 is now something like $699. I’m definitely a REL convert now. They prioritize 2-Ch music listening when designing their subs and that comes through in their performance. 

The more subs the better, but I agree, with speakers like those, tread lightly - only use very good subs.

I don't have speakers of that caliber.  But I do have 5 RELS in a huge carpeted bonus room (entire footprint of a big house) and it took all those subs to make the room.  

At this point, I truly marvel at the tightness of the bass.  And BTW in terms of dialing them in, the gain on the subs needs to be truly low - can't hear any individual subwoofer.  If you can directionally hear it, not dialed in. 

Hey folks, many thanks again for all responses and thoughts…never thought this would get this much attention!

Really appreciate everyone taking the time to respond…

Happy listening to all!

One great sub is better than two so,so subs. But two great subs is better than one. Two good subs together won't go below 20hz any better than a single good sub that won't go below 20hz will. We need to get away from the phrase "tight bass" which actually translates to less deep bass in the real world.

The only reason for 2 subs is if you don't have space for 3 or 4, but you do. The poster who claims you absolutely don't need subs with these speakers is unfortunately ill informed and has obviously never heard a good multi sub set up. I suggest he read Geddes, Welti or Toole

The advice to look at Rythmik or GR Research is solid, a servo system that IMO outperforms most.  @hilde45 mentioned variable phase which is essential for truly dialing in the subs which will exclude most of the REL range. Another drawback of REL is their use of passive radiators which behave in a similar manner to a ported speaker which I also suggest you avoid. A port is another bass source but unfortunately it is frequency invariant and therefore can't be tuned. Home Theatre guys like them but my recommendation is to just stay away for music. I own a REL and a small SVS SB1000 and it handily outperforms the REL. Sorry REL owners, I gotta tell it

To optimise the rig you're advised to measure your installation using REW (free download) I use OmniMic V2 to measure but it is now discontinued and replaced with an improved version available from Parts Express for $599, I think.

The crawl method to locate the best spot for the subs is poor advice although popular. What you will be locating is the loudest bass and that will be from one of the room's many peaks but what you won't/can't hear is the deep null nearby. It's a null, a black hole for music so there is nothing to hear, however measurement will reveal all peaks and nulls and juggling the parameters will flatten this out to a smooth response. The variable phase function will allow more freedom in placement.