Duralast redtop okay as auxiliary for stereo?

use it... but buy a second fuse to put on the power wire near the 2nd battery... and you should buy a high amp relay to isolate the batteries when the car is off. Otherwise they will drain from each other if left sitting for a few days. 200A version bc if you are 2000 watts RMS .... you will be near 200A not 120A.