One per channel. The schematic is of one channel. "IN" is the input signal. For RCA you'd want to ensure the ground connection between source and amp is connected yes. I'm not sure what you mean by this. No schems that I know of but it's essentially a current source fed into a gyrator to buffer the signal from the heater PS. If you are going to be messing with DHTs and don't want to design your own heater supply just buy his regs, they are well regarded among the DIY community. For better impedance matching it should depend on the load or you could have different taps off the transformer for different load impedances.