What is the magic behind 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.

Regarding loudspeaker matching, 805 and 845 output tube designs are available with 40-50 watts/channel. I recently bought a pair of Ming Da MC805-A monoblocks from Coem Audio in HK for under $1400 shipped and have been most impressed. Their 50 w/channel produces formidable and articulate bass from my 91 dB @ 8 ohm 3 ways with 10 inch woofers. No manual/customer support were potential downsides and you must be competent at opening up the bottom and biasing output tubes with 1000 volts nearby. ASL has a model with outside bias meters, occasionally for sale used in the $1500-$2000 range. The upside is SET sound with preserved dynamics at quite loud volumes out of many full size "conventional" multidriver dynamic loudspeakers. It doesn't hurt that NOS 805 tubes can be had for $60-$100. Surely not the "magic" of transformer-coupled 300B amps, but just as surely the SET "family" sound in a far more affordable/versatile package.