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
yes, thinking REL t series.   never owned them but interested if the ability to blend easily with main speakers and bass quality might be worth the tradeoff of lacking deeper low end frequencies....

This discussion thread is from he UK where they tend to have smaller rooms, but it's eye-opening in the range of opinions and experiences it contains.  It's worth reading through...

https://www.pinkfishmedia.net/forum/threads/subwoofer-integration-particularly-with-harbeths.199005/

I’m curious to hear what OP purchased and how that worked out. I have a very similar setup - SHL5+ in one end of a large finished basement. Thinking about adding just one sub - at least for now. How has the experience been adding the sub(s) with the Harbeths?
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.