...but at the beginning it's really hard to play a straight tune, find notes, etc. - so I thought: There's probably a tuning program within my Linux repositories.
And yes there is: "fmit" (Free Music Instrument Tuner) - and it connects to JACK.
Just connect "system.capture_1" to "fmit.input" and you're set. It might be necessary to adjust the threshold of fmit (I was too silent at the beginning, so nothing happend).
It's really nice:
You can see the waveform, the volume as a curve and of course: the tune.
Here's a screenshot from fmit's homepage, but I might provide one with a theremin signal, too:
This makes things way easier - especially on a non-linear model such as the classic "moog etherwave standard".