I don't use a regular guitar amp on stage. Instead, I use an amp modeler (currently a POD XT Live) that can emulate a lot of different amps, and I run the modeler directly into the PA system. As a result, my guitar sound is dependent on having a good PA system. My main band (The Donohoe & Grimes Project) has been using a pair of QSC K10 powered PA speakers for the past year or so. The K10s are awesome speakers. They have 1000 watts of bi-amped power built into each speaker, powering a 10" speaker for the lows and mids and a high-efficiency horn for the high end. All in a compact 35 pound package. They have very flat, clean response. In the past, we have set up the K10s behind the band so we can hear what we're playing and singing. Although this lets us hear the mix well, we can run into feedback problems by having speakers pointing directly at the vocal microphones. To lessen the problem, we cut back some of the high frequencies in the vocal EQ. Unfortunately, this can muddy up the vocals to some extent.
As I said, the K10s are great, but QSC makes an even better powered speaker...the K12. It's essentially the same as the K10 but with a bigger (12") speaker. The K12 has better bass response and more general headroom than the K10.
Since my guitar only sounds as good as the PA, I took it upon myself to improve my guitar sound by picking up a pair of barely used K12s on eBay. Last night was the inaugural gig for the new K12s, and everyone loved them. Not only did it improve the sound of my electric guitar, but all the other instruments -- and the vocals -- also sounded better overall. As a bonus, we now have the old K10s available to use as wedge monitors in front of the band, so we can put the K12s in front of the band (where PA speakers really should be) and we don't have to tweak the vocal EQ to compensate for feedback. The net result is that the vocals are a lot clearer and cleaner for the audience, and we can hear ourselves better onstage.