voltage problem in 400watt amp

Using power transistors near its maximum ratings requires very careful circuit design because any minor oscillation, cross-conduction or thermal runaway issue will kill them. On the other hand, these are very rugged devices and I remember seeing people saying that they were capable to use them with +-70V in a reliable way.

Also, your transistors may be fakes, that would explain the blow-up quite well. In case you don't know what I mean, search the forum for 'fakes', 'fake transistors', etc...
Of course the problem may be simply that the output stage bias currents has increased dramatically with the rail increase from 30V to 70V. Amplifiers with resistive input diff'l tails are particularly sensitive to such changes in supply - their operating currents would more than double sending the output stage bias through the roof!