cherryred21duramax

First Sierra purchase, have been a Ford truck owner and want to try something new. I'm looking at a 2021 3.0 Duramax with the SLT package, preferably with 8" touch screen and Nav. What should I look for when test driving used trucks? I've driven 3 so far and they felt dramatically different, some had a lot of engine noise while others were super quiet. One had a noticeable downshift while coming to a stop, while the others didn't flare when shifting. Some had a very sensitive throttle (checked that they were all in the same mode, put in Normal before test driving) and others had the normal turbo lag you'd expect from a diesel. I'm finding it odd that all of these trucks are between 99-120k miles. Is there something specific that results in people trading these in around this timeframe? Most were 1-owner, used personally, and CARFAX showed "engine/powertrain checked" or "emissions system checked" but doesn't show the actual repair. Looking at both GMC dealerships and independent car lots, is there any benefit to going to a tier-1 dealership (such as Carmax) for warranty options? I have never purchased a used car and bought the extended warranty. Last, what's up with all the trucks having the Treble cranked to +12 on the radio (both Bose and non-Bose options)? Every single one I drove was set that way by the previous owner. Are the speakers lacking, or did I test drive 3 back-to-back with the dash tweeters blown?