Hello Blues&BebopFan. The M-75 is a great guitar for blues and bebop! I played mine as I was transitioning from a flat-top player to styles that demanded more jazz articulation. But I still wanted that hollow-body feel (M-75 has that). It is one of the best guitars I've played. I've only "retired" it since I've acquired an "American Dream" by American Archtop, which is a full-body, floating pickup guitar. And that's why I did not notice the warping of the pick guard. But if I had it out every day, it still would have warped.
The warp probably resulted because the pick guard is black plastic over white. The black apparently shrinks more quickly than the white, causing the pick guard to warp up like a ski-jump. But it did flatten when I treated it as I described above.
Other than that, the only problems I notice are plastic related. The binding (ivoroid plastic) has shrunk, most noticeably in the tight curves at the waistline of the guitar. This is a relatively easy fix for a professional luthier. Also, the heel cap has shrunk a bit and separated from the heel of the neck. Similar repair. We've learned a lot about plastic since 1954.
But the M-75 still plays great! Really, it is a jazz archtop, reduced in body size. No problem with feedback unless perhaps you play at extremely high-volume, though I have not tested it in those situations.
I live in Pennsylvania, USA. Some years ago I was performing at a club along the Delaware River. I went to the men's room and there were snapshots of a women's band playing in very revealing costumes. Two of the guitar players played M-75s! What a riot!
That's it for now. All the best to you.
--Dr Jazz