What is the magic behind SET

It has the magic in front and back and left and right. That is whats behind the SET.

Some designs utilize transformers to couple the driver tube to the output tube which eliminates any capacitors in the signal path. With very, very good transformers this is about as close to "straight wire with gain" as one can get it would seem.

Indeed, my Moth Audio Si2A3 is just like that. Input wire to driver tube to output tube to output transformer. Nothing else in the signal path and its very sweet.

C123666, are these the interstage coupled amps you referred to in another thread? Could you guys list a few? I'm very interested. Thanks!

Wavac is one company that uses DC coupling which also eliminates capacitors in the path. You'll have to search; there are not very many at all. Sun Audio has an interstage coupled model, methinks. Border Patrol offers a beautiful interstage coupled four chassis solution for about 10 large (plus shipping, duty, sales tax); their products rarely come up for sale used. There must be a reason.