i had the same problem and i was very confused because my earlier versions of this amp worked perfectly . but when i used irf 9640 it failed even three hours after working properly. when i changed to irf540n and irf9540n everything is o.k up to now even when using 4ohms at +/-50vlts
Problem in my circuit was the output inductor.when I properly measured it was just 15uH.I added a few more turns and now it is fine.but to be safe I changed the fet to IRF540.