i have a pair of trashed Dynaco a25s that are on a 3 year restoration plan. anyway i had the same problem with the MDF separating at the corners. what i did is rounded off all the edges with a router, then impregnated all the seams/separations with a couple tubes of cyanoacrylate(krazy glue).
should look cool if i ever get around to painting them. there was no hope to stay with the wood veneer look, and i'd rather have them black anyway. i got some nice expensive vintage Marshall style type cloth to do the grills with...
the thing that put me at a standstill was i used some "killz" latex based primer and the stuff does NOT sand. it gums up any sanding discs i put to it within seconds. so eventually i guess i need to strip that junk off and try again
this summer was so cold wet and crappy there was only a handful of days suitable for outdoor sanding/priming/painting, and i'll be damned if i'm going to spend my 4 days of summer sanding and painting ratty old dynacos....