After waiting several weeks for the good folks at Mesa to build it, I received my Express 1x 12 Express extension cab today. It sounds really good and looks cool with the 5:50 sitting atop it. It really widens the sound coming from the amp, although I may try and cover up the back to experiment with how it effects the lower end. I'm planning to cut a solid piece of wood and velcro it in place. If I like it a few screws will do nicely. I don't know if the type of wood I use to cover the rest of the back will matter. If so, recommendations are appreciated in advance.