Hi all,
I‘m totally new to Moog synths. Of course, I admire them for decades but the dream to actually own one just came true recently, a few days ago.
I went with the Sub 37 Limited edition because it was affordable.
And boy do I LOVE it and its sound!
We‘re getting to know each other now.
Yesterday I was playing with the modulation features (MOD 1) and was able to modulate OSC 1‘s volume by switching the MOD DEST switch to VCA LEVEL (and turning the MOD AMT knob to the right).
However, as soon as I switched MOD 1‘s waveform from saw to rectangular (second from left on the MOD SOURCE switch) the volume wasn‘t changing anymore. I‘d expected that it changes according to a rectangular wave but instead the volume didn‘t change at all.
I looked into the manual but couldn‘t find any information that this isn‘t possible.
So is my Sub 37 faulty? Or is is meant to be like this? Or did I miss a firmware update? 
The MOD section’s square and up/down ramps are unipolar. If you use negative MOD 1 AMT to the left, i.e. negative modulation, it should work. This is because the VCA is always at full volume unless you use menu PRESET EDIT 2.5 to set the patch volume to less than 100% - if you do that, then positive modulation with the unipolar sources will work for the VCA.
From this information we can conclude that the actual VCA LEVEL is not modulated by the amplifier envelope (“A”), but that the envelope controls a separate amplitude modulation stage before or after the VCA. Both the MOD sections and the envelopes are software generated, so they both most likely control a single physical VCA, but with an algorithm to give the described effect. But I digress.
The fact that you had the saw waves working, and thus already used negative mod amount, means that the square/rectangle should work too…
What a perfect and helpful reply, DrJustice! Thanks so much!
It’s exactly like you described and it works with negative MOD AMT.
Learned a thing, made me happy! 
(Although I still wonder why it’s not mentioned in the manual…)
Cheers & have a nice day,
Stefan.