I'm not sure I agree that efficiency is not a positive factor when a focus is performance at lower listening levels. The Fletcher Munson curve certainly comes into play but is likely not much of a factor at, say, 60db. In my experience, the more coherent a speaker is the more realistic it sounds at lower levels. TAD with its concentric driver, is such a speaker. Devore may also be good in this regard, although I have yet to hear them.
Two speakers outside the mainstream may fit the bill nicely are both field coils, one from Wolf Von Langa, the "Son" and the other from Songer Audio. The field coil power supply in the Son allows you to control the magnetic field of the woofer and therefore modify the level of woofer damping to get the bass dialed in to suit your room and may be useful for tuning at lower listening levels. Both speakers I referenced are efficient, 92 and 93 dB, respectively.
Ricco, good luck!