I need a subwoofer/sub-bass opinion from you

Hello All--

I have a large room (well large to me).  It's 20' long, 15 wide, and 10' tall.  I run either Klipsch Forte IIIs or PSB Imagine T2s through a tube amp.  I have plenty of volume with either loudspeaker.  

What I want to accomplish is to add sub bass.  I did this previously in a smaller room with a Sumiko S5 (I believe it's the same or similar to the REL T5).    When I had the Sumiko dialed in, the system congealed into a wonderful sound.  There was no noticeably appreciable sound coming from the sub.  Instead, it merely filled out the lowest octave and added space, dynamics, and really made the highs seem less harsh.  

The Sumiko S5 does help in this setup but it's just not enough to charge the room or at least add enough.  

I'm thinking REL.  Thoughts? 

PS  When I ran the Sumiko, it sounded clearly better when I ran it off the amp speaker outs than off my Primaluna's dedicated subwoofer out. 
You can't go wrong with Rythmik, SVS, or Power Sound Audio.  I am happy with the subs I have now (I own subs from all three companies), but if I was looking for a new one I'd take a hard look at SVS' new lineup.  I like the idea of being able to remotely control the sub and adjust it from your listening position.
take a hard look at SVS’ new lineup
Thanks @big_greg, @jbhiller and the rest of you for the dialog. I too am shopping sub configs as upgrade to single REL R-328 in main 2ch hifi room. New SVS SB-3000 looks very promising indeed, given spec and price point - would get dual units if that proves to be the winner.

But I’m kinda anguishing the choices (in a good way):

1. REL S/3 x2 (used, $2500)
2. REL S/3 x1 (used, $1300) + my existing R-328 <= _yes, it can be done: got very helpful & detailed how-to instructions from John Hunter, REL owner & lead designer_
3. SVS SB-3000 x2 (new, $1899)
4. Something comparable from Rythmik?

What do you all think?  Listening area is 2808cf at 18’ x 12’ x 13’h ceiling avg, with one side opening to a 1430cf dining room. The single REL R-328 seems to do fine right now - but I got the upgrade itch, and a hunch that an upgrade will raise the game to whole new level. Main speakers are Nola Boxer v1 on Dynaudio Stand 4.

"Rythmik, HSU, SVS and PowerSoundAudio ... are much better for the money than REL."

@mzkmxcv - why are those brands better for the money than REL?

@big_greg: do you run a 2x sub config for 2ch hifi?  If yes which brand and model?
I run two SVS SB13 Ultra subs for 2 channel listening.  It's overkill, mostly because I don't have enough room to put them in separate locations.  They are stacked on top of each other in between my speakers near a corner.  It would be better to have them in separate spots.  My speakers are Legacy Audio Focus 20/20 (three 12" drivers per speaker).  The subs are dialed down pretty far and blend nicely to provide the lower frequency response that the 20/20s can't reach.  One of these subs would be more than sufficient. 

The SB-3000 specs are close to the SB13 Ultra (a little less power, but 800 watts is more than enough).

The Rythmik F15HP would be better.  I really like Rythmik's subs, very tight and well made.  A bit more money though.  I have two F25's in my home theater setup.  

A great bang for the buck option would be a pair of Power Sound Audio S1510s.  I have an S1500 and it's a nice little sub.  Not as pretty as the SVS subs but it gets the job done.