4" square muffin fans, one on each side of the amp works well for me, using a variac to slow them down to a tolarably quiet noise level. Do not use a noise-generating dimmer for this, not even the motor control variety.
and orient the fans such that they draw heat away from the heatsinks vs. blowing cool air directly onto them, in order to reduce thermal stresses & uneven temperatures
and orient the fans such that they draw heat away from the heatsinks vs. blowing cool air directly onto them, in order to reduce thermal stresses & uneven temperatures