Personally, I don't think the Fender Sunn cabs are so hot. People seem to be really into them, but I have cabs that are pretty rigged and they blew that Fender Sunn cab away. I have an old Peavey 4x12 that I gutted and put two V30s and two Greenbacks in. The Fender Sunn cab sounded like a child crying in the distance compared to that thing.
I have never played through a vintage Sunn 4x12, so I don't really have much to say as far as that comparison is concerned.