Aleph 3 oscillations

A3 ooooooops

Theoricaly shoudn' happen.
Check if the 221 R gale resistors are close possible to the Fets.
You know that theorical roll-off is -3 dB at 100 kHz.Then is the 500 kHz test valid.

Also check for ground loops,especially if happens on only one channel.Check connections I/O and PSU for that.