erm well, I am using the G10 vintage, but in a vox VR30, and it's very nice. I would think the Greenback and the Vintage would be a nice mix, I like the idea of it. Have you considerd the brand new Celestion G10's Golds?
p.s. If you do order the Mongotone I can't wait to hear clips of it, as I am very close to buying the 2x10 myself since you first mentioned them.