AFAIK, Earl is one of the slowest people to get his amps (in some cases a year + on certain amps). That is not preferential treatment at all.
While the Zana Deux may have more inputs and outputs, the Extreme can have those added easily. The cheaper tube compliment has already been mentioned, but if you didn't see it, here it is again. With a tube amp, that's definitely a plus. And last I checked, the posted price of the Zana Deux doesn't even have the full compliment of tubes to run the amp. You still need to buy a 6SL7 and the 2 6c33s.