Upper-mids are normally easily dealt with using absorbers - even 1-inch thick slabs might be enough if placed in the best position, and I certainly wouldn't replace any of your existing absorbers with diffusion.
Upper-mid harshness suggests to me some nasty reflections off nearby objects - look to your mirror points or your desk, and a ceiling cloud can make a huge difference if they are coming from above (it certainly did in my small studio).
Thank you Martin, I wondered if I'd overdone the absorbers as I've got also got a carpet (so I wasn't sure about a ceiling cloud), and curtains permanently covering the window as well.
I've experimented with putting towels over the iMac screen and the desk where it sits and that helped.
Currently the speakers are on foam supports on the raisable desk but I might put them back on stands and then put foam between them and the back of the iMac.
The wall behind me is where book cases with CDs magazines and books are randomly kept, a sort of diffuser I guess. I'll try hanging duvets over them some time.
I'm happy listening on open backed headphones but can't judge track panning and vocal levels very well, that's where the speakers come in.