The interesting thing to me is that the timbre controls seem to have been intended to modify the instrument while it is sounding. An organist can do this by pulling stops, and is similar to setting patches on a Moog, but generally, you play the pitches with both hands and pause to change settings.

I wonder if this was done simply to aid the prototyping, or if the intention was that the performer would be changing the timber during the performance, while the instrument is sounding. This seems to be the effect you hear with the klaxon sound changing into a grunty, nasal timbre at the beginning of the Sackbut Blues.

My guess is that it was a prototyping convenience, but it would seem to have big implications for how the instrument is used in performance. A composer writing for it would have to think about mutable timbre not dissimilarly to planning pedal changes for a harp or even thinking about double-stops on string instruments -- there are practical limitations of the performer and instrument that has to be considered.

That actually makes the music a bit more interesting to me than an early 20th-century ideal of "liberating" music from the confines of catgut and sloppy wet wood and metal.