I need help describing the sound of a SET to a friend in English.

My impression of SET is one has to hear to understand.  Not optimistic it's possible to understand before one hears.

1930's table radio technology! I have four of them! Best used with speakers rated for 100db @1 watt. That means something like an Altec VOTT/A7 or the slightly smaller A5.

Music comes out of a speaker, not an amplifier. If an SET is properly mated to a great speaker (high efficiency, high impedance), then you will have great music.

Like an old table top radio.

It’s like cozying up next to a warm fireplace on a brisk evening. It’s intimate, present, seductive, and memorable. 

SET amps focus on current and the first watt. The tubes add the glow and bloom to the music, and rectification and quality transformers add the push and sag to make it very involving and also softer and more pleasing to the ears.