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.