Subwoofer Recommendation for Harbeth SHL5+


Missing just a little bottom end and slam (e.g 50 Hz and under) I am looking for (2) subwoofers to match my Harbeth SHL5+ speakers.  
I would prefer them to not step on the midrange- sound quality is a preference.
I will continue to run the Harbeth's full range.
Smaller is better for room fit.  
I listen nearfield about 10 feet away in one end of a finished basement. 
I understand the benefits of swarm type systems but this won't be practical for my setup. 
Thanks in advance and peace.   
avanti1960
Rythmik F12 or E15/E15HP2. Check out Dirac Live Bass Control coming out soon in AVR's (and I'm not normally an AVR guy). Looks like an effective way to smooth out the room modes with optimization and without a dba.
I am not a fan of using subwoofers with main speakers especially if it is a Harbeth Super HL5 Plus. I prefer running the speakers full range.

I did try running a PMC sub with the Harbeth and it didn’t work. The sub is mostly unused and occasionally connected to the Home Theater system.