If you are considering the Harbeth P3ESR XD why not go a bit further to the Harbeth M30.2 XD? I have a similar size room and the M30 are fantastic. A bit more presence, great holographic and a mid-range that is second to none for any bookshelf under 1000$ . Would be a good combo with the amp you have. You can probably pick up a pair second hand at your price point.
in any case, if you end up getting a Harbeth speaker, I would recommend getting an open bottom stand , like the Ton Trager, to get the full box resonance. I have tried different stands and it makes a huge difference.
Good luck and keep us posted.