tube amp for ribbon tweeters

You should consider single ended to avoid the upper midrange grunge that can come with push-pull. Also, the output transformer tends to protect the delicate ribbon when turned on and off. It is easy to destroy the foil with on-off transients.
Also, the Atma-Sphere s-30, though probably a splendid amp, is way overkill for the job.
A tube rectified power supply is also desirable to prevent power supply noise in the upper frequency ranges.
The ribbon is probably only amplifying about 10 percent of the power requirement of the audio frequency, that is like a 10db boost in efficiency for the frequency range, a couple of watts would be plenty.
That is why a lot of the tube rectified small console el84 single ended amps seem to be ideal for the job, fairly cheap, too.

Cjfrbw

Which amp would you recommend then?

Offhand, I would recommend one of the Decware Zen el 84 tube rectified models. Some of the older models allow switchable both triode and pentode operation, so you can try both ways to see which way you like better. They also have a volume level, which comes in handy if your active crossover doesn't have one.

WOW! Even new they are very resonable...have you heard it? Sounds (looks) to good to be true.

I have heard one in another system and they sound good. I use single ended el 84 tubes for my foil tweeters, and the single ended el 84 sound the best of many different amps that I tried, varying from solid state, push pull, DHT etc..
The Decware Zen is also not fussy about speaker impedances. You can pretty much sell it for what you paid used anyway, so it is worth a try, I can't imagine needing much more for the high frequencies.