I liked the sound of my bike from the factory. But I like it even better with the SE Street Cannons on it. Made it just a bit louder but noticeably deeper in tone. On my VTX I probably had half a dozen different exhaust before happening on a set of Roadhouse slip-ons that I put on it. That was the last set I ever wanted for it. The FB sounds even better than those did, to me. And with the new cam it lopes nicely once warmed up too.
Your observation of "at least below 80" is pretty much exactly what I was thinking when I mentioned that the X would beat the M8 in the long haul. I'm not an aggressive rider either. I only had my X above 80 a handful of times in nine years, and my FB only a couple of times so far. But from what I recall the X really comes into its own at 70, and is very smooth from that point onwards. My FB at 80, even in sixth gear, for some reason feels like it's making a lot of noise and is not as smooth (feeling) as the X. There's still plenty of throttle left, it's not a matter of straining at all, but I just feel like the X would keep climbing after the FB tops out. I can't recall what rpm the rev limiter kicks in on the X but I think it's about 5400 on the Softails. I'm thinking the X is north of that. From what I understand the HD Touring bikes are set up differently and run smoother at higher speeds.
M8 Fat Boy S Bonneville Salt Pearl