Yes, a wire from either pin 1 or 8 (they are jumpered in the Prinzetone) for your red probe jack. 

You don't *have* to add a black jack, you can just touch the black probe to any exposed ground connection, a ground is a ground.

The reason people have bias check points at both tubes is because if the tubes aren't perfectly matched (few are), you can see which one runs hotter and adjust for that tube. For bigger amps with 6L6GC and EL34, a 4mA mismatch is considered acceptable, but for 6V6GT's and EL84's it can mean the difference between 70% and 90% dissipation, so you want to be able to check for it.  You could even add separate bias pots for each tube and never have to worry about whether they are matched or not.

Good advise to turn off the vibro 
Think I'll add a grounded point as well, as the chassis is painted.
Another bias pot would be a good idea actually, but where to run the wires then (goes to the last22k 1w resistor on the main board as is)?