I'd say that those are different enough that it's not exactly a fair comparison. The Zana is larger, and at least has some ventilation at the base of those tubes. Does the Extreme have any?
I'd respectfully ask that you go back and look at the pictures and notice that the trafo in the outboard supply is a 12.6V/4A - 6.3/8A unit. You and I both know that this is not the HV supply. Mikhail may have built other Extremes where he's moved the HV out of the main chassis (that sounds like what he told me at Canjam) but that is clearly not the case here. I'm guessing that this Extreme didn't start life as a Platinum and therefore doesn't have the Plitron trafo, but I'm guessing there.
The 5998 is my personal favorite in this type of amp, but the right 6AS7G sounds pretty darn good for $10 tube. The 7236 can also be good and not break the bank as well.