Small footprint subwoofer

If the Kef is more than you want to spend two REL T5/x are about the cost of one Kef. Two SVS 3000 micros are slightly more than the two RELs. The SVS at 23Hz goes deeper than the REL at 32Hz but my in room response is a good bit lower than the 32Hz spec.

I have two SVS 3000 Micros and like them, and enjoy the control app on my phone. I have not compared them to the competition, fwiw. 
 

Best!
 

I installed a Kef Reference 8B for a client recently... I was worried it wasn't going to hit low or have enough power for a large room... I was wrong, not sure how they make two 8 inch woofers move that kind of air but holy smokes that thing is legit, it can go very low and very loud if you want with authority.. It is $8k retail but wow, its awesome and looks beautiful and compact, its magic how much sound that little sub can put out

In the more affordable category the old Sunfire HRS 10 or HRS 12 is another great compact sub that I have installed dozens of and they hit hard, deep, tight, and clean... 10" is 11.5" cube, the 12" is 13.5" cube, now that Sunfire is gone, they are still making them but under the Speakercraft brand, one of the best subs for the money and the very compact size

The SVS SB-1000pro was a consideration a while back but still think would be too big for my room especially going two subs. I get the performance aspect of it but I really don't need anything lower 30hz since this is just a 2 ch music only system.

There's no such thing as "too big" unless you don't have the physical space.  Those are small subs, smaller than the one you have.  They don't have very much power.  They have level controls.  You can reduce the output to integrate them with your speakers.  They're only going to be "too big" if you don't adjust them correctly.

If you can let the subs coast and aren't pushing them hard, then they are less likely to distort.  If you're going to use a sub, why not get one that is capable of producing all the bass that's in some recordings?

@jastralfu  True but my situation and room size, I dont need overwhelming bass. I do however have a room node at around 60hz but that can be fix with by integrating the sub with the proper settings on the sub and placement.  Even at that, the xo point would be lower then the node.

@big_greg I have the STF-2 currently and their about the internal volume as the SB-1000pro with the exception that the STF-2 is rectangle and a little taller. The SVS might be an option but it is a 12" but that is ok...I guess.