Full detailed sound at 30 - 40 - 50 dB

Tubes rule! Or not, but I didn’t know tubes were better for low volume (I learn something from Atmasphere about once every 4 and a half days), but I prefer tube amps anyway so I don’t recall the difference since I’ve had a tube amp in the rig for so long…I have noticed better low level sound since I put a pair of efficient horn speakers in the rig, but the tone issues with lower levels are always there so you’re going to need loudness compensation for that anyway if you want the highs (and lows) to get to your earballs…hense my Schiit Loki usefulness. One thing about the Loki is it’s single ended RCA only so balanced "pre to amp" users have to run it through a tape loop or something (although not all preamps have those these days). Maybe the Schiit-heads next EQ (Loki Maxi?) will be balanced. Also the immediate slam and general snappy response of a great tube amp (mine is a lowish powered Dennis Had Fire Bottle HO), along with the inherent tonal "rightness" (an official audiophile term), makes tubes at least "seem" balsier than most SS amps at higher volume when paired with appropriate speakers.

Play complicated jazz so the baby becomes smarter.

Note that creating an unusually quiet environment for a baby can condition the baby to not be able to sleep without utter quiet, sometimes not a good thing. I don't imply anyone should expect a baby to sleep through excessive loudness, but I've seen this happen with one of our grandkids…in a largish house everyone having to whisper if the baby's sleeping…actually a first in my wife's and my experience (raised 3 kids between us), and because we're "all knowing" grandparents, our expertise is, of course, unquestionable, although we could be wrong about everything as all kids are different.

Get a Schiit Loki EQ gizmo…I have one and it’s really useful for exactly what you describe, and pretty much mainly what I use mine for when needed during quieter late night listening. It’s also relatively inexpensive for a well made thing. Note I have very efficient horn loaded speakers and the phenomena of low volume frequency perception is still a thing, although perhaps to a lessor degree than with other designs.