I think I can help!
I also found the mic sound a bit too bassy.
I'm not at the keyboard at present so I'm only working from memory.
Bring up the mic settings page, select multi effect set(I think its called that), scroll through the various effects until you find parametric EQ, then select TOO BRIGHT (I think thats what it's called), then go back to to the first mic page and switch the multi effect ON. It works independently of the multi button on the face of the keyboard.
You can permanently save that setting perhaps as the custom panel mic setting or in a panel memory.