If you have pre-amp, just set the Bass and Trebel to 0 i.e flat sound..

Control the tone through pre-amp or HU...

As for gain, set AS LOW AS POSSIBLE, but not 0 as it will give -ve gain, which is SOFTER than original sound...

Other reason to set it as low is to avoid your amp overheating....

try setting it (near) max and see how long (hours) you can blast...

yupe... setting the amp to high gain might get you louder volume (good for SPL competition) but at the cost of loosing SQ and over heat very fast...

I just like to confirm this ...

If set gain correctly (usually less than 50% max volume) IT WILL NOT OVERHEAT when played at long hours..

If the gain is set high >70%, it take less than 1 hr for the amp to overheat...

Usually when overheat, the amp will cut power to the block, if a bit severe, it will burn the fuse, and in extreme cases it will destroy the amp completely...

This apply to all classes of amp....