Full detailed sound at 30 - 40 - 50 dB

Get a Decibel meter and measure the sound level in your room with EVERYTHING off. My bet is you'll see numbers in 40ish range. So regardless of the system, 40ish dB listening levels is basically background music at best. As most have suggested, get a headphone and crank it up to your heart's desire.

I recently built a pair of back-loaded horn speakers using full-range single drivers and had a similar challenge as the OP since they are in a system in an upstairs room that I’m gradually converting into a small listening room. So low volume listening levels are important. What I discovered after experimentation using my own (SS) gear and borrowing gears from my friends was that a low powered tube amplifier with a very low damping factor - high output impedance, and using basic speaker cables, produced the best sound quality at low volume. The trade off however is the lower bass gets a little boomy at higher volumes. Ironically, the amplifier that gave the best result was a very moderately priced Chinese unit with only 8.5 wpc. Next best was a Decware integrated. So like everything else in life there’s always a trade off.