I'm thinking you may be running out of steam from the amp or maybe the speakers by using too much low EQ.
The Low Boost switch is adding 8db at 20hz which is way low for live bass and no bass cabs that I know of can even handle 20hz...also, where on the scale is the 40hz graphic slider...?
Have you tried the amp with different cabs...?
Manual is here: [url="http://www.crateamps.com/pdf/manuals/BT350_OM.pdf"]http://www.crateamps.com/pdf/manuals/BT350_OM.pdf[/url]