The 2 double receptacles on your 20 amp line equals one 20A dedicated line.
It's fine to run an amp plus preamp on the same line; the 2 components will draw less than 20 amps. It would not be a good idea to run monoblocks on the same AC line due to the high current draw.
It's fine to run an amp plus preamp on the same line; the 2 components will draw less than 20 amps. It would not be a good idea to run monoblocks on the same AC line due to the high current draw.