Aside from bypassing the fan controller and directly connecting to a 12v DC line inside the receiver you would need to disconnect the leads of the fan from the board and route them to an external power supply capable of handling the amperage requirement of the fan. If you choose a lower voltage power supply the fan will run slower, but quieter. Unfortunately it also appears the Technics SA-AX6 is void of a switched outlet (at least it is on the European version) and it doesn't have any 12v triggers or control connections, so you would have to manually power the fan on and off.
Again, this all assumes you DON'T bypass the switch that activates the fan inside the receiver when you reach a certain volume. Unfortunately Panasonic did not use a thermally activated switch in their receivers, it is solely dependant on the output level in relation to the level control. In theory you could connect a variable source to an input on the receiver, such as a CD player with headphone output, and attenuate the volume with the source and leave the receiver at the level necessary to activate the fan.