Full detailed sound at 30 - 40 - 50 dB

Are you a male or female? If the former I want to know how in the heck did YOU have a baby"? What did you dilate up to before delivery? I'm sure your wife or girlfriend is pleased to know that fact. Now regarding your question about low level listening, have you considered room treatment? 

Get a pair of Quad ESL's! Probably the most revealing speakers ever!

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.

Lowering ambient noise as much as possible is indeed a first requisite. In a new house you can specify sound proofing glass for the windows, muffled ventilation, a quieter heating system, but in an existing house these are all hard/expensive to do. Even, so there may well be possibilities.

The headphone suggestion is a good one, it cures a lot of woes and can give you some great listening sessions, also keep in mind that the speakers are like an open window to your upstream components and are not going to cure any of their short comings. I myself have my system set up, in near field, such as to render all the dynamics and presence I need at low volumes. Most my listening sessions are late at night into early morning, while grandson is asleep and can honestly say that I don’t miss the higher volume but do crank it up when I can but absolutely don't miss the higher volumes when I can't.