How about higher sensitivity speakers? could this not be another and possibly cheaper solution?
I can’t see how higher sensitivity speakers would help (other than that they would sound different just as any othe speaker would, and one might sound better than another). What higher sensitivity would do is mean the volume control would set be even lower for the low sound level the OP wants, and any noise from the power amp through the speskers - hiss and possibly hum - would increase in volume relative to the music, so actually decreasing sound quality. (If the problem was at high level listening, then higher sensitivity speakers might help, depending on the cause of the problem.)
I hadn’t picked up on the passive bi-amping when I wrote my first response. I have never really understood passive bi-amping, even more so with two amps feeding a three-way speaker, and it is possible that something undesirable is happening, Maybe compounded with amps having non-identical characteristics. As others have suggested, it makes every sense to try with a single amp, using the best one - which is almost an instant thing to do, simply disconnecting the other at the speaker terminals and refitting the links, and if that fixes the problem then end of story.
But if the issue then remains, it seems to me that there are three possibilities, assuming that all sounds great at higher sound levels:-
1) the speakers are not reproducing well the signal fed to them when at low levels - if that is the case, then the only answer is different speakers. Maybe some dealer demos at those same low listening levels will help you decide if the issue is confined to your speakers themselves.
2) the power amp is behaving poorly at low level- if that is the case, then the only answer is different power amp. However, this seems unlikely as problems with power amps tend much more to be with high levels.
3) the music is losing its frequency extremes, due to the reducing sensitivity of the human ear at high low frequencies as sound level reduces, the loss of bass being particularly evident. If this is the cause of the problem, the only solution is an artificial boost of the frequency extremes to compensate - loudness compensation, as I described in an earlier post. This may be anathema to the purist audiophile, but then it could be argued that listening at levels lower than music would be at a live performance is false, and all it is doing is compensating for the consequence of so doing. A simple manual approach is to use a digital signal processor inserted in the signal path, and factor in some boost until it sounds right to your ears. A few seconds of Googling will find you the charts showing what the ear’s fall-off in sensitivity looks like, the boost needed would be seeking to produce the reverse curve, though something vaguely approximate is likely to suffice. If you can borrow one you’d soon discover if it fixes the problem.
or just turn the volume up...