I have a Marshall Greenback cab and recently starting mixing speakers in it.
First, WGS ET65 and Ret30's sounded alright, but the 65's are just too dark for my liking.
Currently I've settled on Greenbacks and Ret30's in an X pattern. The mids are great and the the lowend is solid and tight from the V30 clones. I like my speakers to have some degree of brightness to them and the Greenbacks add that nice highend crunch. The Ret30's have a nice top end too that the real V30 lacks. It also shaves off some of those gnawing upper mids and replaces them with a more pleasing and controlable amount of mids. Next to a real V30 they sound very similar, but slightly different.