I also have P3ESRs, in the desktop system in my study (driven by a 2x100 watt Quad 405-2). I think they are wonderful, and I know nothing better in that size. But they have their inevitable limitations: they lack deeper bass, and they cannot pump out enough sound in a larger room. Out of curiosity I decided to try them in our large living room, driven by the 2x140 watt Quad 606-2 that I normally use there. It still sounded great, but its limitations were also a bit more obvious. So I added the B&W PV1d subwoofer (plus DSpeaker Antimode 8033 room eq) from my main system. Bass extension was immediately of a different order, of course, and integration was perfect. The experience of such deep bass seemingly coming out of these little speakers was uncanny. The combination also produced a larger sound bubble, but not quite large enough to be completely convincing in this large room. For that I would clearly have needed larger main speakers (i.e. the M30.1s). So that is the main difference that you will experience if you move one size up.
I have never heard the P3ESR and the M30.1 back to back, but by all accounts they are very similar in character. My son is saving up to buy a pair of M30.1’s (he is currently borrowing my LS3/5as) and once he has bought them I will be able to compare directly, but not yet. He did consider the P3ESR but decided he needed a bigger speaker for more dynamic music.
I have two more observations to make about your system. From my own experience I would suggest that it can be improved considerably for little money by using a DSpeaker Antimode 8033 room eq unit. My second observation is that Harbeth’s designer Alan Shaw clearly prefers powerful solid state amplifiers (he designs them using various Quad amplifiers). Harbeths have a relatively flat impedance curve, but even so solid state amplifiers are likely to give a more neutral/flat response that many tubes. From my own experience I also observed that these speakers like a bit of power. My 2x100 watt Quad 40-5- would be my minimum choice. Of course the M30.1 is more efficient than the P3ESR but even so, 70 watt is not that much. At
the upcoming Bristol Hifi show
Alan Shaw will demo the P3ESR with a 2x100 watt Yamaha AS701. I recently gave my son a 2x250 wat Yamaha P2500S to be used with his planned M30.1s.
