With 400W RMS and a 10-12 foot run, will 8AWG work fine? There's really no chance of upgrading in future so if 8 is ok, then 8 it is. Would there be any audible difference in 4awg vs. 8?
RMS doesn't matter as much, because amps pull different amounts of current to deliever the same performance(efficiency). 10-12ft, about 80 amps is the cutoff for 4/8 guage.