...you tell me what changing volume levels does to improve the record?
Changing your volume knob's setting is a way to bring that recording closer to the actual experience of the live event.
Example 1: Our 15 minute per side harpsichord recording. It is recorded at full available groove modulation due to the short length of the musical composition and the lack of bass inherent in the instrument. At a live performance, let's say the average perceived volume at my seat is 72 dB.
I play the record with a preset volume control. Due to the LP I am presented with, I have a harpsichord blasting away at 90 dB. That hardly strikes me as desirable if one is trying to closely recreate the live experience.
Example 2: We have a LP of a 50 minute Mahler symphony. At the concert, the average SPL at my seat is 90 dB. Due to the inherent limitations of the LP format, the LP was pressed with lower groove modulations. (This is NOT because the engineer did a "lousy" job. The physics of LP production required that lower level.) When I play back this record on my fixed-volume level stereo, I get a 75 dB playback level. That is substantially softer than what I heard live.
The solution for the vast majority of music listeners is to turn down the volume somewhat when listening to the harpsichord recording and to turn it up when listening to the full symphony.
The quality of the record is just what it is. There is nothing we can do to "improve" that recording once it has left the pressing plant. However, one variable we do have control over is adjusting the playback volume to match that of a realistic concert. If you choose to listen to loud music at a low volume and soft music at a loud volume, that is certainly your prerogative. Of course, there are certainly times where the volume setting will be a good match for your LP. However, to not play a particular record at all, or to listen to it at an inappropriate volume simply because of a volume mismatch strikes me as unnecessarily robbing yourself of otherwise enjoyable experience.
In my opinion there is nothing magic about a particular volume setting on a preamp since the output volume of source material varies so widely. (This is also true of CD, open reel, FM broadcast and other music sources as well as LPs.) Having music play at an appropriate volume for the piece in question is important to me (and many others I suspect) and it is something over which I have control. It seems you choose otherwise, but that is certainly your option. Just remember to enjoy your music.