Adam's reminded me that (despite my earlier post) there really is a Themodist test roll - and that I've got a copy! Came in a batch lot, and I never played it... the music didn't seem very interesting.
The Angelus system is an on-off form of accompaniment, rather than the graduated-throttle that Aeolian used (a slide valve to gradually cut off the direct air flow) or the fully-governed Hupfeld system (where there is a smooth decrease in the accompaniment level as the theme sliders are moved).
That means Aeolian and Hupfeld both have user-controllable accompaniment levels according to the position of the theme slider, while Angelus (and several other systems) have a fixed accompaniment level, which must be set correctly to give quiet playing without missing notes.
That's the purpose of the cam on the regulator - strengthening the spring by turning the cam will make the accompaniment louder. I like this means of adjustment, very intuitive and faster than the more common spring-and-turnbuckle that most systems used. Both bass and treble should be adjusted similarly.