While I agree with what has been posted so far, that with proper placement they should sound very good, if they are SMGs, they are getting very long in the tooth, and there's a good chance they are starting to delaminate, and/or have tweeter wire corrosion issues. Even the SMGa is now over 20yrs old, I suspect the last SMG was made well over 30yrs ago!
Magnepans, particularly earlier, pre-quasiribbon era models, used glue that tends to give up around 15-20yrs out, and aluminum wire for the tweeters that was prone to corroding, causing tweeters to fail. Some do seem to last longer than others, I have a pair of SMGas that had the main wiring pop away from the mylar several years ago, I just pressed it back in place, smeared a little thinner over the existing glue to stick it back, and they're still working today, but that's the exception.
They are simple two way speakers, with about 3/4 of the area on one side used for bass, and the last 1/4 as the tweeter. If you listen closely with your ear up near the grill, you should be able to hear sound from both the tweeter side, and the woofer side, if you can't hear any distinct sound from the tweeter area, and the fuse is good, the aluminum wire has probably failed. Magnepan offers kits to repair them, as well as in house service, at reasonable prices.
Bottom line is, yours most likely are not working correctly, and if they were repaired, you'd most likely be impressed with the improvement. They are not known for "sparkling highs", or deep bass, but generally they don't sound dull... they had should sound pretty flat from 50hz-18khz, which is better than most large bookshelf speakers do.
