I don't know about "image density" but as an apartment dweller I wanted excellent sound at low volume levels. I found that I could get this with sensitive speakers. Mine are around 93 db/w/m. You know how some systems need to be cranked up a bit before they come to life - well, I did not want that at all. The other advantage of sensitive speakers is you can go to single-ended triodes - at least, I can in my room.
The combination I have now - Coincident Super Eclipse II and Welborne Lab Laurel IIx with new Western Electric 300Bs (I recently replaced my well loved TJ meshies) is very satisfying at low volumes.
Regards,