(I posted this in the "Vintage" section, but it may fare better here)
I have 4 M3's from the early 90's (wedge-shaped black resin housing) that all seem to have very little high frequency output. Two I purchased and two were given to me, but they all perform the same. I compared the sound with the same receiver into Infinity SM-82's and Bose 191's, and aside from the SM-82's "brightness", the M3's were truly lacking in high-frequency sound. I haven't used the M3's in two years due to moving, and they have been stored in the open on elevated shelving and never damaged. Removing the tweeter screens did not seem to help. They're nice little speakers; is there a replacement tweeter? Could the crossovers be degraded over time?