Did someone say AMPEG? :D
All of my amps are Magnavox-era Ampegs, so I tend to have a fairly narrow perspective on some things...
Bear in mind that Everett Hull absolutely hated Rock'n'Roll music. Which is ironic considering that his amps pretty much defined what rock sounded like in the 70s. What gives these classic amps their killer sound is that they were designed to play well clean, with lots of headroom, using robust components. So when you push 'em hard, they sound like God has come to party! :cool:
Celestions are made to break up and add their own voice to amps that are pretty much one-trick ponies. When you push them just nicely into the 'sweet spot' they deliver one particular kind of sound. Which is why Marshall and Fender folks love 'em. And why they have no business being in an Ampeg cab. Why artificially limit an amp that can do anything the other amps can, plus a whole lot more? The original Jensen, Altec, Eminence and CTS speakers were chosen for full-range, undistorted performance, letting the amp speak for itself. And Ampegs deserve to be heard, uncoloured, straight-up, and balls-out!
So go to the >Weber VST< website and browse through the spec sheets and the user reviews. Then send an e-mail to Ted Weber. He'll answer in person! How often do you get to deal with The Man himself? After sorting through some reviews, and a few e-mail exchanges with Ted, I picked a 15" ceramic-magnet California (Altec-style) with a paper voice-coil dome for my VT-40 salvage custom job. It's amazing! Every detail comes through, and I get the full benefit of the most versatile tone controls around. The 12" ceramic California would be just killer in your GU-12!
OK, now, you're maybe gonna get sticker-shock from the price, but give it some thought. I bought 1 15" speaker instead of replacing 4 flood-damaged 10" speakers. Not the same situation you're facing. Meanwhile you can check out the Jensen clones available from Weber. The VST stands for Vintage Speaker Technology. And the man knows what he's doing. I've been working in some pretty exotic technical jobs for the last 30 years, and he managed to impress the hell outa me! So stroll on past the woulda-coulda-shoulda stuff. You didn't settle for a Fender or a Marshall. You don't wanna sound like every body else. You got an Ampeg, and they always sound as unique as the guy who's plugged into them. So don't settle for one of 'the usual suspects' when you choose your speaker.