Then maybe I should upgrade output transformers in my VAC Avatar with custom ones ? This is a crazy idea, I think.

@inna Could be... The output transformer is the most critical part of any tube amplifier. Its only crazy if you think so... You could easily wind up (if you see what I did there) with a garage full of prototypes trying to get it right.

The higher the amplifier power the harder the transformer is to do. It is exactly this reason I designed OTLs instead and prefer smaller tube amps if transformer coupled.

The EL34 tube is sweeter and gentler with the detail than many KT tubes.  Most KT tubes sound hifi- ish. I’ve found the EL34 to have better resolution also over the KT tubes.

@lowtubes In this statement the thing you are not taking into account is the effect of the output transformer. For that reason you can't draw a conclusion since that is such an enormous variable! 

I don't doubt the EL34 can sound better because the output transformer typically doesn't have to handle as much power. For that same reason the EL84 sounds better than the EL34 (probably also because the EL84 is so much easier to drive...).

it's purely a matter of design

@inna That's the answer.

If you want to hear a really musical KT88 based amp, the Harmon Kardon Citation 2 (properly refurbished) is a good place to start.

It makes a difference too if the amp is pentode or Ultra-Linear and also what class of operation its set up for. You can't get away from the simple fact that the larger the output transformer is, the more critical it becomes and the harder for it to make bandwidth. IMO this latter issue might be the most important.