Guitarists with too much low end is my vote too, second would be your tone (scooped mids), except you mentioned you're running the amp pretty flat.
Acid test? If you stop playing and hear plenty of low end, and it doesn't sound thin, its the guitars.
Hard to cut through on bass. There's not a whole heck of a lot of treble content in the bass, easily obscured by the much more prevalent treble of the guitars, and when they also boost their lows, it acts exactly like you cranked your low end up (and mistuned your E string...

)
Its not that they overwhelm your low end, you've got way more than they do. The problem (IMO) is with such a small amount of treble in your sound, their low end easily overwhelms your high end that gives you pitch/definition to your sound. So you sound muddy, indistinct, can't tell what notes you're playing.
That and they're too damn loud to begin with, of course..
Randy