subwoofer for Harbeth P3ESR

Personally I wouldn't use a sub with the P3ESR and listen to it as is for the sake of simplicity. If I want more bass or scale I'll just consider a larger model in the Harbeth line if the room can accommodate. Having said that I've tried a sub with the P3ESR and it blended in quite well. Bass augmentation usually works better with fuller range speakers though.

I use a Rel Stratta III with mine in a quite large room and it works very well.  

A sub will add more weight to the P3ESR.  I can't think of any reason not to use one.  I have mine set up in a nearfield system, about 3 1/2 feet from my listening position.  I use a SVS SB12 NSD. 

How big is your room?  That should dictate which sub(s) (multiples are better if you have the space) to look at.

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.