Just my take on the stock audio for my 2019 Custom. It actually sounds pretty good....with eq and using your phone (not FM radio). EQ is the secret. The bass pumps pretty well and it is doesn't break up easily. Clarity would be my issue and probably fixed with speaker upgrade. But it isn't that big of an issue. Compared to the last 2 vehicles I drove (2014 Equinox Pioneer system with sub and 2017 Grand Caravan GT with 9 speaker system and sub) the basic Silverado stereo sounds better. Just FM radio is only okay, but I use my phone 99% of the time so I don't care.
As for the eq, it goes against what "audiophiles" like to do, but I found the system to have too much mids. I set the bass to +4, and cut the mids leaving the highs flat. In fact, I was going to add on the factory sub but really don't see a need to do so (and I'm a bass player for my weekend gig).