I'm trying to use my lexicon G2 with a preamp triaxis-mesa boogie conected to a mesa 2:90 power amp.(marshall cabinet 4x12) wich the best way to connecte them, becAUSE I'd try to do it, but the sound is not good...please help...

Your in luck, I run Lexicon here in Los Angeles. I also own a G2 rig. We can get you hooked up in no time.

What is the I/O configuration on the back of the Mesa? Is it a stereo or mono effects send?

There are three basic groupings of presets within the G2. Settings for Amps with Effects Sends, Amps with input only, and Stand Alone Presets.
They will all function within whatever setup you have, they just won't sound their optimal best if the preset doesn't match your set up.

Let me know