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.
