Welcome. Since you are dealing with an AC signal, you can one fuse either Pos or Neg side, but most people choose Pos. To calculate fuse size, take the power (in watts), say 200, and divide by the speaker impedance (say 8 ohms), then find the square root of the result. 200/8=25, √25=5, so in that case I'd use a 5A fast blow fuse. You can use either ATC or ACG type fuses and in line holders.