You can set your gk growl on gain a, and set your clean with gain b. On the mb 500, its a little more limiting because there are 2 masters but only 1 gain. On the 500, if you want your growl on a, its going to be on b
On the mb500, you have the boost though. Typically, you don't want the gain set high on the GK heads...... very shrill and brittle sounding.
