Just for grins.....what cabs do you use with your iAmp 600? I'm running a pair of CxL-112s and I find that I get a TON more volume out of mine if I crank the knobs up equally to about +5 instead of 0. Mostly active basses as well.
dcr
A CX-310 (JDong says it's an 8ohm load after the voice coil warms up, plus I play with a fairly light touch).
Also, the tone of the amp just sounds meatier to me with the gain cranked.