...Speaking of Music Man amps. They are known for their high plate voltage vs. reliability.... but few seldom mention that they also operated at exceptionally low (on generic guitar amp standards) screen voltage. The same little detail was also the key difference of prototype Ampeg SVTs (that failed catastrophically even with extremely reliable and rugged output tubes) and later SVTs that didn't (which no longer had tubes of equal ruggedness but simply much lower screen voltage). Something to ponder at. 
Yeah, I noticed those voltages were like 1/2 B+ right?
If I understood correctly, this is to keep plate-screen grid voltage within maximum limits. With the help of the cathode transistor keeping the cathode at a rather large, positive voltage (negative relative to the grid, which is fixed-biased), so the plate-cathode and control-grid voltages are within limits...
Yeah, that's it! This is a fixed-bias stage that's driven from the cathode instead of the grid.
Would then the transistor be simply a common-emitter stage, with the tube working as it's "collector resistor" and at the same time, the transistor working as the tube's "cathode resistor"?
If that's it, it's SO much easier to understand!