The Boost knob is the growl knob, along with setting the input gain high. You can make them totally clean by setting those 2 really low, but it does lose you some output, which you make up by cranking up the master vol.
There is always a hint of GK sound in there but you can get rid of the growl by setting the master full-on, then bring up the input gain just enough to reach your desired volume. You might have to take the Boost just a touch above off to get there, but much of that, and you'll start to get just a bit of that sound in there again.
The Boost knob shines in a band mix. You can manipulate it at certain settings to dirty up the amp by playing harder or clean it up by playing lighter. You might not like it solo'd....that's fine too.. Some have accused them of being "too clean", others "can't get rid of "that sound"".