[ben]

From my experience I would suspect the cart. All the 8TAs I have seen in the last decade or so had deteriorated suspension. I use a slightly newer Sonotone 3509 cart (previously stripped and rebuilt by my good self!) in mine, and it sounds great.

My second call would be to check the HT voltage, as already mentioned. The speaker could conceivably have a scraping coil but check the other things first.

I suppose one way of checking the cart would be to wire the 31D's output socket to a suitable(1 meg?) input on another amp and see if the sound is distorted.