Hiya,
Take *all* of 1kHz out of the graphic and you'll probably lose a lot of that feedback on your 58s...if there is any low rumble, take all of the 250Hz out as well (the vocals and sax don't need it, you can always boost the LF on the keys channel if it needs more low end retaining)...if the rig sounds too 'thinned out' with this extreme EQ, gradually bring a little of whichever frequency back in.
I don't know those particular Peavey cabs but would imagine they sound pretty scooped - in addition to what I suggested for the graphic you might want to drop the treble EQ and bass EQ on each channel by a small amount.
I know it might feel like you're cutting everything but doing this on the channels in moderation could be a good move especially if you don't have loads of experience with the graphic.
Remember you can always use one cab, the mixer and one 58 to spend a little time at home or rehearsal getting to understand what frequencies have what impact on your sound with that specific gear - this time will be well spent! I guess the feedback you're getting is mostly HF as your instincts were to cut the treble, so get someone to speak in the mic and listen especially to how 2kHz, 4kHz, 8kHz on the graphic affects the top end when either boosted (at low volume...very slowly and in moderation, careful not to hurt anyone's ears!) or cut.
Hope this helps!
Tom