This is a topic close to my heart: I listen to low volume classical ~11-12 hrs/day and so it's important to me that I actually hear the music. From long trial and error, I found 2 interrelated factors that help with this:
1 - Use relatively inefficient/insensitive speakers. The two pairs I switch between now are both ~83-84 dB at 1M. It takes some juice to drive them, which means more play on the low end of the volume pot of whatever preamp I use; and
2 - Use sealed/acoustic suspension speakers. Both pairs I have are sealed. Somehow this lets more clarity flow from the speaker at low volume. There's no port to hear or deal with.