I'm by no means an expert, so this might be silly.
You're asking the tubes to dissipate 360W, Wikipedia says a single tube can do 25W max, so that's gonna be a lot of tubes.
At 600V, the equivalent resistance of 600mA is 1k Ohm, which at 360W should be doable as a regular resistive load. Then if you hook your dynamic load in parallel to this 1k resistor, the pass element only has to do <100W according to my sim. Maybe use a bit higher resistance so you have some headroom, 1.1k would work up to 650V and keep pass element still <100W.
The 2SK1317 mentioned can barely do it within the specified SOA, though if I made this circuit I guess I'd put two in parallel to be on the safe side. It's also specified as a switching transistor, not sure how reliable the DC SOA is.
Then again, what do I know, maybe a dumb idea?
In any case, do you really need it to go all the way down to zero?