For me, a speaker sounds "flat" when there isn't enough treble or high frequencies. Without enough treble, the bass seems exagerated or boomy. (to me)
Before I bought the M80's, I had the same speakers for over 20 years. In the last 4 to 5 years, I experienced the same change in sound with those old speakers you did with the M3's. Coincidentally, it was at about the same time when I noticed that I had a difficult time holding a conversation when in a room with lots of backgroud noise. Diagnosis - a bit of high frequency hearing loss.
Now I don't mean to be cheeky, but have you thought about this possibility?
