The blade tension can also sometimes affect tracking, so you might tinker with that. Also, perhaps drop a string over the top wheel, thru the guides to maybe see if someone left out a spacer or something on the bottom wheel shaft assembly, with the wheels not in the same plane. The tires should have a very slight crown on them; haven't looked at the Grob, but most replaceable tires have a small crown machined into the wheel surface. With the guides moved away from the blade, the blade should run pretty much in the same general position on the wheels.What tracking adjustment? i don't see anything on it.
Edit: Never noticed that teeny little thumbscrew before. Ok, i have the top wheel tracking nicely on the crown, the blade is running in the guides, the lower wheel, it tracks out on the edge still. I probably need a lower tire at least. I can shove it into the bottom of the guides when cutting, but the blade works out to middle when done cutting. It's good enough to keep it in the guides for now, i can get my 2 exhaust jobs done. That thing is SLOW on 60hz slowest sheaves.
For cutting round stuff, I use a Palmgren bench vise to clamp the part (part flat on the table), to keep the part from rotating; those vises with all sides machined allow clamping round parts on the end or over the top while keeping the part on the table surface).