I would have thought that the closest thing to Plus-x in the MP range would have been MP Plus-X, not Double-x, but I know they are all a bit different, and it depends on what qualities you are comparing. I shot about 800 feet of Double-X back when you could still find short ends. It's nice, to me it seemed closest to the older tri-x, like the the tri-x I shot in the early 80's. They have improved away a little of it's character since then. Anyway I would love to see them do something that would be very easy to do: 35mm 36exp. Factory loads of MP stock. It would be great to see some of those other emulsions out there, easy to get and use.
Well they still make it in 400' cans, and rakpak? Sells re-rolled spools of it here, just search.