I thought I'd add to the list and say I have the same problem as the rest of you. I too, like my music loud, and after volume level 22, all the way to 38 sounds identical to me. There is literally no difference. Just FYI, I do not have hearing loss, I can hear my wife whispering things about me to my kids from the other end of the house!
I've tried turning on the "loudness" setting, which seems to do nothing at all, and also playing around with the speed adjusted volume setting. Like a previous poster, I found the best results doing the same thing he did, moving the balance and fade more to the front of the truck. But it's still not nearly as loud as I'd prefer it to be.
I do agree that this seems to be some sort of automatic noise canceling effect in play, as the volume seems to fade in and out from time to time as I'm driving (even with the "speed adjusted volume" turned off). Sometimes the volume goes dramatically lower and I crank the volume up to compensate, and it's already at 38.
I wish there was some fix, manual adjustment or hack we could do to these things to increase the max volume, even just a little bit... The crappy cassette radio and speakers I had in my Buick Skylark when I was a kid was louder!