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.
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.