I ended up auditioning both, and have gone with the P3ESR. While both speakers predictably have strengths and weaknesses, let me begin by saying that the assertions in the above post are ludicrous. I don't mean to suggest that the other speakers mentioned aren't contenders, but the rest is hyperbolic nonsense.
Now, with regard to the two Harbeths, I have chosen the P3s primarily because of what some earlier contributors have noted, namely that they are more open sounding than the M30, and this is especially important in my current, less than ideal space.
I have little doubt that in a room with fine acoustics, and with the right associated equipment, the M30s would be first-class. The highs and mids were superb, even in my space, but they lacked the openness of the smaller speakers, and at times sounded 'boxy'. They also required a more specific and narrow listening position in order to be fully appreciated, while the P3s are quite flexible in that respect.
Having said all of that, I am not as enthusiastic about the P3 bass response as some others on the thread. My expectations were realistic, but I have been a bit underwhelmed. To be fair, I haven't yet done much experimenting with positioning, nor have I tried a different set of spikes, which I will do. So it is still early, and my opinions may evolve.
I do like the speakers quite a bit, but am not sure that they will be with me for years to come. For some perspective, I was more impressed with the bass response of the previous (small) speakers that I was using, a pair of Von Schweikert VR-1, which cost all of 350€.
In any case, thanks for the earlier feedback, and if you have any further thoughts or questions, fire away!
Regards,
Tony C.