Thanks for your reply @chrisAre you ... Out.
I'm familiar with FM, but never done anything in real life to address it. Yes, with my (tube) guitar amps, I play them at around 85dB+ (where they sound best.. lower means missing bass and character). I can have them at that volume presumably because as you say, the sound isn't being thrown right at my ears, but bouncing around the room before getting to me. Even in this instance, I know I'm sensitive to the 1-4kHz range and too much does my head in.
With the FM9/Adam combo, by "boring", I mean that (regardless of monitors or headphones) at lower volumes, the amps all sound muffled and samey, with no character (all the marshall types may as well be the same amp, same for fender etc). If I turn my headphones or monitors up to the level where the amps sound good (and much more unique), I'm certain it's a volume that is harmful to my ears. The patches were all tuned at loud volume, maybe 85, even 90dB (before my secondborn came along haha).
Let's take the headphones for a second, to avoid the acoustics of the room for a second... Is it a "done thing" to apply a global EQ at lower volumes, bumping up the treble and bass a bit, so that you can play "live tuned' patches at lower volumes without messing them up when you go back to playing loud? That's the first thing that comes to mind...

I'll still need to experiment with the headphones as obviously the tweeter setting won't change that.