You were right. It was the casing that was loose. I tried crimping the tabs you circled but it didn’t help much so I put a couple of blobs of solder on the pot case and thread.
That did the trick no more wobbly knob.
Oh and it looks like the Digitakt and Octatrack do have the pot for volume. A10k dual pot.
I love a story that ends with nice results. 
Maybe the unit was worked very hard in the past? Not positive of any other reason for the deformation of the metal to the point it couldn’t be casually tightened by the bending trick, but good thinking on the blob of solder! Glad this worked out for you as well.
If you treat the volume pot gently going forwards I’m sure it will be fine. Another thing you could consider is adding a cheap in-line variable attenuator style volume control. They sell them on Amazon etc. And it would just add an additional, inexpensive, plug-in way to adjust volume when added in between the output jack and your headphones.
Probably unnecessary but at least something you could consider.
And nice confirmation on the pot values. I was pretty sure but still, good to have it 
% confirmed.
This is a very wholesome thread