I just want to make clear three points:

1) Tube Dude's null test doesn't have anything to do with looking at specs. Tube Dude's null test tells you if the amplifier is doing what it was designed for in the first place: the output must be only an amplified version of the input, nothing more nothing less;

2) Tube Dude's null test avoids (unmeaningful) specs;

3) The beauty of a mirror image (like the one from an accurate amp) only depends of what one puts in front of it.