It's an interesting question, about the maximum practical driver plate current. Here's a calculation I've done but not documented before:
You have 20mA to play with. In drivers I've always done equal reg and driver current, so 10mA is available (you could do more, with less in the reg, if you use C4S driver plate load)
Now you want the driver plate at around 2/3 of the regulated voltage, for good and symmetric headroom - let's say 300v regulated and 200v driver plate. The "beam resistance" is then 200v/10mA or 20K. Typical operating point is Rbeam = 5*rp, so the plate resistance must be 4K or greater. The 6C45 is out.
Obviously you can vary the assumptions, this is a scoping analysis not an exact calculation.