I convinced the other guitarist to lift his cabinet to ear level and the difference in sound was immediate. We could hear ourselves again!
Yes!! Finally someone has actually listened to this advice and surprise surprise it worked! As dmills says, the ultimate sound engineer's dream is a small amp on a stand pointing across the stage. (Or eles it's a band with no guitarists )
Good luck with the gig on Saturday - if you haven't got foldback then amp positioning becomes critical. Get them high up!