Have you ever tried using SMD resistors for MOSFET source resistors?
Chip Resistor - Surface Mount | Resistors | DigiKey
They are rated 3W, for 0.1R source resistors the power rating is sufficient. The power rail fuse rating is only 7A so if 7A goes through 0.1R it will dissipate 4.9W. And that is the maximum power. For normal music playing, the power will be a lot less.
My worry is that I don't know if the power rating can only be realised by soldering the resistors on large copper areas to dissipate heat. If only one side of the resistor is soldered on some small area of copper (track of 5.5mm wide and 10mm long), will the power rating be much reduced and the resistors get burnt?