Full detailed sound at 30 - 40 - 50 dB

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.

I want to come back to the preference for tubes with high output impedance playing at low level. Why could this be? I have two explanations. The first is that a particular tube amplifier may have a non flat frequency response emphasizing low end and top. This is not infrequenctly the case (see the Stereophile response graphs for many of them), and would act as a traditional loudness control. The second explanation could be that underpowered amplifers actually compress the signal. They depress the peaks, and this has the effect of raising the relative level of the quieter passages, for more 'detail'. This is then a crude version of the digital compression software as used by FM radio stations to keep the signal within the limited dynamic range of the FM format. Yes it works, but is it realistic? No. But then, no low level playing is a realistic representation of the real thing. Enjoy the baby - my kids have now reached the age when they like to play the audio at it's limits....

But if the total is low, the  components must be even lower (or theoretically at most equal).