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
i picked 2 REL t9i.  best subs for audiophile quality music i have owned and heard. 
integrated perfectly, does not color the midrange and flat down to 20 hz room response.  plenty of power despite the power ratings.  
I’m having great success with a rel 812 and it will only make your mid range sound better if dialed in correctly. Good luck