The thing I've noticed about Class D amps, GK and the old Ampeg Portabass Class D units, is that they don't like having the master overdriven. An engineer at Ampeg told me to turn the master up full, then use the pre as your actual volume control. I've found this applies to GK as well.
+1
I was pleased with my GK head right out of the gate.
But when I learned to do this my amp came alive.
I run my master around 3 o'clock and use the input as my volume control.