I have a theory about this that might explain everything. Just a guess but let me float the idea. I love the sound system in my car. I've even listened to some dance music because the bass sounds so good... not normally in my wheelhouse. Applause from Lady Gaga sounds REALLY good on that system... definitely not normally in my wheelhouse.
I used to do high end stereos in my cars in the '90s like many of you and I have experience with amps and large capacitors. And, my wife's previous car was an Infiniti G37 with the premium audio (we both have Model 3s now). The stereo in the Infiniti was good but the premium audio Tesla stereo is definitely better.
In my 3, several times, I have had the bass suddenly disappear and the audio sounded flat. Sometimes it was a few seconds and other times it was for an entire drive and it went back to normal the next drive. We all know the car is monitoring the voltage in just about every circuit. I'm sure that includes the 12-v power from the 12-v battery. What if when the car senses a lower voltage (but not low enough to cause a failure to start or send an error code), it cuts power to the amp(s) to protect the 12-v battery? Maybe those who are reporting a big difference with the Ohmmu battery had an original 12-v battery that put out slightly lower voltage than what was expected by the car, but not low enough to cause any problems starting. Or, if the voltage sensor on the circuit was slightly out of spec, or both. Then, the higher, more stable voltage from the higher quality 12-v battery eliminated that problem and everything sounds the way it does for the rest of us. That would explain those reporting an improvement and also those who are skeptical that the new battery would make any difference.
I'd be curious if someone else with a factory battery would meet up with 12adrock, or someone else who has reported a sound improvement with the new battery, to see if there's any difference. Maybe you unlucky few just got a factory battery that was subpar but not bad enough to cause a failure.
EDIT: BTW, we both have 2018 LR RWD Model 3's with the factory 12-v battery.