In what application?
Is there truly going to be 400v right on the tube, or will the plpate load be connected to 400v B+? In which case the question arise, what voltage will be on the plate. And that question begs another, what is on the cathode? The voltage specs on tubes are for the most part the voltage across the tube itself, not necessarily the voltage to ground. That is to say if a 12AX7 has 400v on the plate and 300v on the cathode, the tube only sees 100v.
And that brings up another factor - heater to cathode voltage. That one is important. If you have a 12AX7 in a cathode follower where the B+ is 300v and the cathode is at maybe 150v, if the heaters are left near ground, there is 150v between cathode and heater, and that could be too much. In those cases a simple DC elevation of the heaters solves the problem.