Hello Egoben,
Although I have interest in the Harbeth line of speakers, I have not yet had the opportunity to have heard the P3ESR's in person and have a few questions for you if you do not mind.
In regards to your post; ".....fills the room perfectly, big sound, enough bass, punchy, with great balance....." I like how small mini monitors can seem to "disappear" imaging wise, but due to their small size, they often seem to miniaturize the size of instruments, if you will. Like toy violins, cellos and toy pianos, etc. Do you feel the sense of the instrument's scale is lost at all with such a tiny speaker as the P3ESR?
Also, you mentioned that they are easy to drive with your tube amp. I'm a tube lover as well and am curious which tube amp you are using with your P3ESR's?
What size room are you playing these in and how close are you able to place them to the wall that is behind the speaker without suffering any sound degradation?
Thank you for your help!