That's safety Earth and looks fine - I'm talking about throughout the schematic (not familiar with this specific kit) where different components are connected (referenced) to ground, you need to make sure all those joints/connections are solid so you don't have any leakage. You should also have a connection from ground to your metal chassis, linking the board to your safety Earth. This connection is often part of your power supply.
I don't know if you can see the last pictures of my measurements with the new multimeter that I posted, but I decided to solder the whole amp again. I've just finished the main circuit board and these are the measurements I got (these are with the left 300B tube broken):
tp1: 0.8v
tp2: 0.8v
tp3: 96v
tp4: 95v
tp5: 0v
tp6: 0v
tp7: 8.7v
tp8: 8.7v
tp9: 165v
tp10: 163v
tp11: 220v
tp12: 221v
tp13: 332v
tp14: 332v
tp15: -41v
tp16: -76v
tp17: -46v
tp18: -79v
tp19: 404v
tp20: 380v
tp21: 5v
tp22: 4v
tp23: 404v
tp24: 382v
tp25: 0v
tp26: 2.9v
tp27: 6v
tp28: 18v
tp29: 408v
tp30: 404v
tp31: -99v
tp32: 164v
tp33: 12.6v
tp34: 60v
tp35: 6.3v
tp36: 6.1v
tp37: 12.4v
tp38: 301v
tp1: 0.8v
tp2: 0.8v
tp3: 96v
tp4: 95v
tp5: 0v
tp6: 0v
tp7: 8.7v
tp8: 8.7v
tp9: 165v
tp10: 163v
tp11: 220v
tp12: 221v
tp13: 332v
tp14: 332v
tp15: -41v
tp16: -76v
tp17: -46v
tp18: -79v
tp19: 404v
tp20: 380v
tp21: 5v
tp22: 4v
tp23: 404v
tp24: 382v
tp25: 0v
tp26: 2.9v
tp27: 6v
tp28: 18v
tp29: 408v
tp30: 404v
tp31: -99v
tp32: 164v
tp33: 12.6v
tp34: 60v
tp35: 6.3v
tp36: 6.1v
tp37: 12.4v
tp38: 301v