This thread got me thinking. Klipschorns were mentioned as sounding good at low volume. I would agree they can be captivating, although not a lot of bass. I think the loudness curve has something to do with it. But the speaker can have flat on-axis response. It just needs a total power response that has a dip in the middle, and I think that's what the k-horns do with that big beamy mid-horn, and that little tweeter up top.
So perhaps a smaller speaker with an 8 inch mid-woofer that's crossed over kind of high to a small, wide dispersion tweeter would have similar low volume effect.