Full detailed sound at 30 - 40 - 50 dB


I love the nuance you can hear when listening to music at loud volumes, but unless no one is home, it’s not considerate or feasible to listen at such high volumes. Plus I just had a baby so everyone is always home and volume levels are limited to 40ish dBs. 

Any recommendations for getting the most detail at these volumes? Additional gear or recommended integrated amps?

My NAD 7175PE has a loudness button which boosts the treble and bass a little, and that certainly helps things in the Kitchen. 

My living room amp is a Rega Brio-R which doesn’t have any tone controls. 

Any thoughts? Thanks!
leemaze
Also rather interesting that you should point out the relationship distortion has with harmonics and how you consider "weighting".
There is always more to it...

We all know about Fletcher-Munson. The Fletcher-Munson curves show where our ears are most sensitive, which is birdsong frequencies. This is because birds are the first indication of a predator in the vicinity and their calls activate other birds to spread the alarm. It does not matter if you are predator or prey, if you can't hear birdsong you're likely hungry or eaten.

The harmonics of a good number of instruments happen to fall into this region which is up to 7KHz. So the instruments that are making the most energy in music have harmonics that fall in the range to which our ears are most sensitive. This is likely not coincidence, but it presents a daunting challenge to engineers if the goal is to get audio systems to sound like real music and not an audio system! 

Traditional engineering tends to ignore this, although we've known the Fletcher-Munson curves for decades.

One example of this is the use of loop feedback in amp and preamplifier designs. Feedback is known to bifurcate the input signal and thus generate harmonics of its own while also suppressing distortion. For this reason feedback usually contributes to brightness in all amps in which it is used; this brightness is caused by distortion rather than frequency response error. Our ears use the higher ordered harmonics to sense sound pressure, so any addition to these harmonics (or their presence where none was before) is easily heard, despite not being all that easy to measure.

The audio industry is a long ways from acknowledging this fact so we can expect amps to have a tendency to brightness for some time to come. One of the few areas where there are deviations from this norm is in high end audio. Its a weird world...


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@atmasphere 

Take a look at the distortion curves of a typical solid state amp. At a certain lower power level, the distortion is at its minimum and as you go to lower levels the distortion can be significantly higher! This is not a given with tube amps, even push-pull amps (our amps are push-pull but distortion linearly decreases to unmeasurable as power is decreased).

So that power level is about 5-7% of full power with most solid state amps. If you've ever heard that bit about the 'first watt' being the most important, that's because its all about distortion (or in this case, the lack of it). BTW, Nelson Pass has a set of solid state amps called the 'First Watt' amps and they are so-named because they diverge from the usual solid state amps in that they too are very low distortion at low power levels. Their designs are also quite minimalist (even simpler than and not unlike tube amps in that regard) although they don't make a lot of power.

If you want good resolution so you hear everything (including good bass) at lower levels, this is what you have to do: tubes or a low powered solid state amp like a FirstWatt.

Which First Watt would you recommend to the owner of high efficiency speakers (101db, 16 Ohm) ? Currently using push-pull tube amp designed (OEM) by Trafomatic Audio for WLM Acoustic back in the days. 
I recommend you move the baby into the audio room and acclimate the kid to your hobby now. I learned to fall asleep to a bunch of dudes wailing on guitars in the next room...and look where it got me...posting bad advice on Audiogon. :-)
Congratulations on having a baby. One of life's great blessings.