The plot thickens...
I am very close to making a decision, but have received some strong, contrary opinions on the Harbeth forum. Not that the P3 isn't held in very high regard – it is. But that the M30 is a better speaker overall, and that the advantages of the other are limited.
To complicate matters, I currently have a choice of either speaker at roughly the same cost.
hmmm...
You will certainly receive contradictory opinions since everyone's taste and listening preferences are not the same. I have listened to all Harbeth speakers in the line. My least favourite speaker is the M30. No doubt the tonal quality is very rich, but the overall sound is thick and shut-in. All the other speakers P3ESR, C7ES3 and SHL5s are airier than the M30.
If you like an airy and open sound, you are better served with the P3ESR. But if you can listen to both M30 and P3ESR, that is of course the best option.