Marshall VS40V- Scoping the output (or not)

[Jazz P Bass]

O/k.
I have a VS40V /Model 8040 on the bench.
This is the one with the TDA1514A output IC.
The amp came in with R66 (10 ohms/1/2 w) toasted (one part of the output pin stabilizing network, along with a .22uf cap)
I reolaced the resistor & the cap.
I also changed the "bootstrap cap (220uf/ 25V) to a 220/100V.
The amp works fine.
Now, how do you scope this thing?
The black output wire is NOT ground.

[g1]

Scope ground to chassis ground, looks like same grounding through low value R like Fender used to do on Pro 185 (or whatever ) etc.