The problem is that I have not heard a high efficiency speaker that
sounds good. More efficiency usually means more coloration, thinner and
unstable sound. Horns do not image well and the sound seems to originate
from behind the speakers.
That sounds like a setup problem to me. Horns can image as well as anything else if you use their directivity to your advantage. Early reflections from side walls can be interpreted by the ear as harshness; so with horns you can prevent the side walls having such an influence by reducing the amount of reflection you hear.
I run horns at home and their imaging is pinpoint. Plenty of depth too. I would not association greater efficiency with coloration, but it is true that more efficient speakers can be more revealing: you really want your ducks in a row or else the speakers will show off what's wrong! My speakers are 98dB and go down to 20Hz; they are anything but 'thin' :) Their first breakup is at 35KHz so they are quite smooth too.
Put another way, there's no downside to having the speakers easy to drive until bass output suffers at efficiencies north of 100dB. At that point you might have to have subs (and subs can be a very viable way to do bass if you have something called a 'Distributed Bass Array'; google 'Audiokinesis Swarm for more on that....).
I'm not saying that a 1960s horn system is the way to go here; as far as that goes there are a lot of speakers from the 1960s that are pretty colored. I'm also not suggesting that you get something that is 105dB or the like; if you can get something that is in the mid 90s for efficiency and also a higher impedance that will mean you need less power to do the same sound pressure. For example, if your speaker is 88dB, and the new speaker is honestly 95dB, you'll need less than 1/4 the power to get the same sound pressure, and if the speaker is properly designed you won't lose any resolution or have any greater coloration- in fact it may sound more detailed and less colored simply on account of the fact that the amp is making less distortion.
Pay attention to impedance too- higher impedance loads on any amplifier result in less distortion. You can see this in the specs. A simple way any speaker manufacturer could make their product sound smoother and more detailed would simply be to make the speaker higher impedance. High end audio isn't about sound pressure so much as it is about nuance. Getting drums right is going to get other sounds right too- you get closer to the music.
