Kevin is just plain wrong-amps are at their heart the simplest of electronics- not preamps. Amps do nothing more than regulate the energy coming out of your wall.
The opposite is true. Read everything I've said and I can tell you that you can make a DECENT tube preamp for $1500 that sounds damn good. Cary made one called the AE-3DJH and it benched better than most. Tube preamps are SIMPLE and that's why manufacturers are over-charging customers with big empty boxes.
Now you get to the details. What kind of regulation, rectifying the AC, hybrid or not, dual mono. But I'll be the first to admit big diminishing returns and that's exactly why doing a level matched A/B will be less telling.
Tube amps are completely different, and it's harder to cheat. The differences are much greater between the good and bad. And they can be bucks up and bad. Amps don't "regulate power from the wall." Output transformers are the key, and they are half art, half science. And expensive....which is why companies looking to maximize profit use cheap (and small) power AND output transformers. They are the single largest cost.
As to the A/B test, if you want double blind I can try to hide the wires. I would expect the more expensive amp to benefit from NOT being double blind, as people will be impressed. As to which tubes, the idea is to compare stock with stock. I'm not going to upgrade the tubes on the PrimaLuna. It negates what this is about. Would the PrimaLuna benefit from dropping in $400 worth of KT150's? I suppose. But what good does that do? It muddies the water.