Subwoofer Recommendation for Harbeth SHL5+

You didn't mention your budget, but I'd concur with the recommendations for Rythmik's sealed subs if they fit in your budget.

I don't know why you wouldn't want to add a sub(s) to the Harbeths.  I use 4 (two Rythmik F25 and two SVS SB13 Ultra) and it's the closest my system has come to live music.  I know you don't want to run a swarm, and two subs properly positioned and set up should enhance your listening experience.

The Harbeths sound great on their own, but they are relatively small speakers and you're going to miss a lot of the foundation of the music without a sub.

+1 big_greg.
Avanti, you've been here long enough to know Greg's suggestions are a result of first hand experience and sensibility. 

I would add, although I have no experience with one, his choice of SVS Ultra offer a usable level of equalization and extension sub connectivity that would provide you with a subwoofer system that will easily integrate with most any future main speaker selection.

I used a distributed bass array for a brief time years ago. While I fully understand their usefulness I found proper placement of two subs provide my room with nearly the same benefit. Have fun with it.  

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....