I was able to drive some more today while running errands, and you're spot on. With the radio off and paying attention, I can hear the DFM kick in under 55 or so, and I know it's the DFM because I hit L9 on the shifter (which deactivates DFM) and it went away immediately. And then over 65 the exhaust note becomes more apparent again, didn't matter if it was in L9 or not, but when going over 75 it became less noticeable again. So my observations match yours, the frequency or resonance of the engine and exhaust between 65-75 is when any drone is most audible. I didn't think it was obnoxiously loud, but I haven't road tripped for hours yet either.
In a perfect world we would've gotten active exhaust where we can turn it off and on via the Sport setting on the dash like some GMs, but alas they decided not to offer active exhaust on these SUVs. I'm just going to chalk it up as a necessary compromise at this point, because it sounds great running around town, I love the sound when accelerating, and it has a great rumble at idle, but the tradeoff is apparently some resonance between 65-75.
