Turn down the Volume!

@amosm  If you don't want to hear your car rattle, turn up the radio. This is called masking. The same holds true of your highway noise. Play vinyl which has a higher background noise level and turn up the volume till you don't notice the highway.\

Your only other option is to move. I 

I know of no house that is absolutely quiet. Mine certainly is not but, it never bothers me until I am taking measurements then I turn everything in the house off.

I suspect that the reason high-efficiency speakers tend to sound more dynamic than low-efficiency speakers has to do with thermal compression.  For any given output level, low-efficiency speakers have to run more power through the drivers, with a larger portion of that power being dissipated as heat vis-a-vis  high efficiency drivers.  That heat actually increases the resistance of the voice coil and this reduces the power that can delivered to the coil, thereby reducing its sound output.  Hence, there is more of this sort of compression of louder signals with low-efficiency speakers.  That is why high efficiency speakers sound more lively at any given sound level.  I think they sound particularly lively at low levels.

Higher ambient noise levels mask micro dynamics, hearing the full measure of micro dynamics contained within a recording is what makes the music come alive and creates engagement. 20db differences in ambient noise levels, lets say from daytime to quiet night,  within any particular room will greatly affect satisfactory macro dynamic or volume level of system. If one wants to have satisfactory low volume level listening, lower that ambient noise level!

Certainly, the type of music played also has bearing on this. Classical recordings with their generally higher levels of dynamic expression will be more problematic in the noisy environment, rock far less.

I tend to agree that high efficiency speakers and tubes perform better at low level volumes. Tubes, especially directly heated triodes I've found to have greater micro dynamic expression at low volume levels vs push pull and SS. High efficiency speakers, especially horns, expose the full potential of tubes in this area.

I agree - avoid ambient and background noise - but NOT the ambient sound in the recording, which is there for a reason, like a band playing *together* and not each player on their own, the "good ambience" which is often pro-tooled away in modern remasters.

And adjust your speakers (work with this). And some - not too much - treatment of the room. the lower your volume, while still hearing the music clear and good - the better.

Another underestimated factor is the RELATIVE volume, or gain, in your system. If you have two or more gain stages, and can adjust the volume on these, it pays to pay attention to which volume settings that give the best sound. If your system chain is tuned in this way, chances are, you can turn the overall volume down, and still have good sound.

Note that, in my experience, this relative volume tuning varies a lot. Generally if the source is turned too low, the music sounds dull and insipid. If too high it becomes too loud, insistent and harsh. However it also depends on the quality of the gain stages. So for example, for many years, I ran digital music from a pc to active speakers in my home office. I discovered that including a tube headphone amp between the source and the speakers gave better sound. Instead of two gain stages - lowly solid state in the pc, and similar in the active speaker amp, I now had three, a higher quality component in between - and discovered a new rule of thumb - make the best gain stage carry as much of the load as possible.

I downloaded the app for the Spectrum audio analyzer. Works great for very basic measurements. I use it all the time and find that I rarely listen to anything above 85db. As long as the system can handle the peaks, that's plenty for my uses.