Hiwatt experts please chime in...Mark Huss ?

[g1]

G-one, actual signal (after passing 68k input grid resistors) into V1 grid is 10mV, and signal leaves plate at 600mV (please see post above).

This may be true, but you still have 170mV at stage 2 grid, which means from input jack to stage 2 grid, you have a gain of 1. Sure doesn't seem right to me. It would sound about right if the whole tone stack was in there too, but according to the schematic, it is not.
Make sure this schematic is correct for your unit.

[J M Fahey]

G-one, actual signal (after passing 68k input grid resistors) into V1 grid is 10mV, and signal leaves plate at 600mV (please see post above). So V1 looks fine also.

I still think that if you inject 150mV signal (what frequency?) at the input jack (which one?) before 1 or 2 68K resistors (not shown in the schematic) you can not have just meager 10mV at the grid they are connected to.
No way.

That alone accounts for a 15:1 loss

If you didn't have that huge loss, you could drive amp to full power with 1000/15=66mV , much more reasonable.

I would check that first and foremost.

That said, the schematic looks weird:

1) unused triode?

2) cathode follower used only as a voltage divider to bias the PI with no other function?

Weird.

[TimmyP1955]

One of the 68k is bad.