Full detailed sound at 30 - 40 - 50 dB

Get some good headphones.

+1

The quiescent noise floor in my listening room at best is 30db (measured). Personally and IMO, anything music wise lower than 50db wouldn't be worthwhile to listen to from an enjoyment perspective. Background while having conversation about something unrelated to music perhaps. 

Any recommendations for getting the most detail at these volumes?

Yes. Assuming you want to use speakers and not headphones, sitting closer to the speaker by a few inches can be helpful. In my case I like to listen at times in the low 90's, but usually find myself backing off to the mid to low 80's as it gets into the late evening. At one point I positioned myself about 8 inches closer to the speaker (I'm 7 feet away now) and the 8 inch difference was enough for me to lower the decibel range (again measured) by about 2.5. Even a 2.5 db decrease in decibels can be helpful at times.

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.