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(1) Hi-Fi setup: High Voltage B+ (500v) and Large Load (420k), to minimize distortion and maximize voltage transfer to load (this is not a power circuit, so voltage is more important than power transfer efficiency). Although higher voltages raise risks, they give more horizontal loadlines, which means current remains stable (think CCS). Headroom is not paramount here, because typical input signals will be played at low to mid-level volumes, to further limit system distortion, and protection against high voltage swings can be built in via -db pads for input of stage.
A lower bias point is selected, to better center swing in zone of maximum linearity.

(2) Universal Soldier Setup: Slightly Higher Voltage B+ (400 v range) Here is the mid-zone. Reasonable voltage and current excursion is expected, and more current can give both stability and a current source for subsequent stages which might need draw. a bias-point of -2v gives a nice centering for a balance of headroom and linearity.

(3) Guitar Maniac Setup: Here overloading is expected and harmonic distortion (non-linearity) is actually desired. Lower plate load gives a nice steep 'dive-bomb' loadline, while lower plate voltage (330 B+) protects tube from HV shorting, and allows pushing tube into cut-off safely. The higher bias point (-2.5 to even 3.5) is chosen to give maximum headroom for wild guitar antics and easy sliding into non-linear 'sweet-spots', without driving tube into grid-conduction, which is a non-musical type of distortion.