It sound as though your FETs/amp is oscillating, it could be in the VHF frequencies. Build a RF diode probe and use it to feed your DVM, use a 1000pF coupling cap so it will ignore audio frequencies, which you would hear from your loudspeaker. Now you can measure and see if there is some RF volts about your circuit. If there is, then a 100 ohm resistor in series with each FET gate lead might be enough to kill it- wire the resistor as close as possible to the FET. It could be that your supply lines need to be decoupled to RF on the PCB, use some .1MF on both supplies to earth.
Frank