Hi Robert,
without trying to blatently advertise, we are on the verge of release of our own in-wall, on-wall speaker system to the general public that has been designed for accuracy over blast your head off levels. However, being phase accurate to a considerable level does help travel where phase inaccurate designs can falter and become muddy once you leave the drect firing line of the drivers.
These are not going to be the cheapest speakers on the planet by any means but they come as a fully active system where the active crossover/amplifier will be a separate unit. They utilise ridgid carbon drivers with the physical frontage of the speakers being approximately 600mm high by 500mm wide but the depth front to back is just 48mm. There are no defined figures at this time that can be quoted and is some respects, these are not always relevant. Inneficiant speakers can take a high powerred amp to drive them to attain a said SPL (Sound Pressure Level) where as an efficient speaker can take far less grunt to achieve the same SPL. Plus, passive crossovers generally do not reproduce in the same way as a carefully designed active one.
One interesting fact has been made far more clear since investigating this line of speaker system, and that is, i can now weedle out the good from the bad recordings in my collection. the bad sound the same as they always did but the good are now allowed to show off and its amazing what you dont know your missing until you hear it.
PS: Musically, my own tastes vary somewhat as i sit here listening to Classic FM and since readin your post, recalling one of the best concerts i ever went to by a subtle little band called AC/DC.
