IIRC the shield on a DIN connector was always intended to be the chassis
connection, with pin 2 optionally being an isolated signal ground. Quad
were rather naughty when they connected pin 2 to chassis in the 303.
Keeping it separate would have allowed more flexible connection without
introducing hum loops. Linking the shield to pin 2 is fine for low level
inputs, but it shouldn't be done when linking to other equipment.

Modern electrical safety earthing might allow the 303 to be powered from
the 33 as the mains transformer appears to include an earthed shield
which would isolate the secondary from the primary. That would make it a
Class II (double-insulated) appliance. Of course, the shield would have
to be capable of carrying any fault current from the mains until the fuse
blows. Also, all mains leads (& probably internal mains wiring) would
need an outer sheath to provide double insulation.