Another cooling point worth mentioning is that an amplifier fan cover shroud will multiply a fans effectiveness many times. Otherwise, the fan mostly circulates air around the fan perimeter and pushes or pulls air a very short distance away from the fan. Ideally you would like to see the fan encompass the amplifier's entire heatsink surface area.
Most any thin, rigid material (.25" or so) can be used to create a fan shroud, such as wood, ABS, aluminum, plex, a cutting board, etc. The shroud would be the identical footprint of the amplifier and elevated above the heatsink by .5" with 'Z' (double 'L') brackets. The fan would be mounted in the center of the shroud.
Off on a bit of a tangent but related to amplifier heat....there are two seperate functions inside every amplifier. One is the DC to AC to DC switching power supply which is outside the signal path in most topologies. The other, the second, is the audio booster so to speak. As you halve the load resistance (impedance) you will increase the power on the audio booster side. But efficiency drops as the load becomes lower and with a drop in efficiency the ratio of heat generated to power produced to the speakers can get ugly. Now, on the power supply it is a fixed output with a definite ceiling. Ohm's Law no longer applies to the power supply side. It's forgiveable to load an amplifier down on half the channels but loading the amplifier down to the minimum on ALL channels will definitely tax the supply. The result? Voltage from the supply sags. And the amplifier works even less efficiently as the voltage drops. It's possible to load an amplifier down to get a 60 percent increase in power that can generate as much as 4 times the heat! So the initial system design and power management is a big part of it. Sure, fans can help dissipate heat but at the core more heat is an indication of wasted energy.