A sub is a great idea, dare I say, essential with smaller Harbeth speakers, unless you do not want or need a fuller sounding experience. I used a REL sub with my Monitor 30's several years ago and it really allowed the Harbeths to shine. You'll need to take the time in setting up the sub so that the frequencies roll off appropriately to make it sound right, but it really sounds great once you get it dialed. Room size plays a big role in this decision. In a larger room you can make small speakers sound really great with proper implementation of 1-2 subwoofers.
