Well I've been trying not to weigh into this discussion but... While I've nothing against highly efficient speakers and have often thought it would be fun to play with that kind of setup along with set amps etc it's not what I've got at the moment.
Currently running Eggleston Works Andra 2 - and I find I listen to them at all volumes, certainly often at louder volumes, just so enjoyable letting than sing but equally my typical morning routine is a coffee reading the news or something and music playing quietly.
I have noted on more than one occasion to one of my very few hi-fi friends how lovely they sound at low volumes. Certainly no lacking at all in any regard and particularly no lacking in bass.
So while I absolutely endorse looking at highly efficient speakers if that's of interest I would suggest that if the only reason is because of a belief that only highly efficient speakers will achieve what you want at low volumes then I suggest that's not necessarily the case.
I just tested my typical low level volume listening and it's at an avg of 53db. If I'm honest I expected it to be higher, in my mind I was agreeing with previous posters that 50db is too low. Well I guess I just proved myself wrong!
So point of my text for what it's worth is consider broadening your search, I don't see why any good speaker can't achieve what you want.
E.

