Mark - I do have a EV12L in a EV spec type cab. It actually works well - sonically - when the amp is at higher gain settings. At lower gain settings - and it takes a bit to saturate the speaker - its kinda edgy. But either way, the thing is so frickn loud that I have to walk away from it. I have thought about this, too, in how the Dumble guys like this cab/speaker, and have decided that here its the MV thing that lets it work so well. Eh, and the MV thing doesn't work quite the same in the Express for my taste.
On another note, I did plug a Pure64 cab into my Express, running some Austone speaker, don't know what it was actually, and it sounded fantastic. Aren't you running one of those speakers, the Austone brand? Man, I have heard the Scumbacks and they sound great, no argument, but I know the next single 1/12 box I try is going to be with one of the Austone speakers. I just have to get this over with, try one. Eh, and it could be that cab, too? Maybe one of Jim's speakers would sound just as good in the same cab. Yeah, hard for me to ignore the actual cabinet design I think.
Glen - Yes sir, you have got all the head smarts and tone experience to run about anything (including a metal speaker cab?) with your amp and make it sound good without hurting anything. !! Which is not to say it doesn't sound fantastic, because I'm sure it does, but hey, you're kinda in the Express-Zen-Master catagory, and not typical, I think.

Most people stall out when fixing a mistake that they've made. Why?