SO you have lost the B+? WHy on earth is this any more difficult to figure out than a tube amp? Transformer, rectifier, cap, resistor, how is this different from the same circuit in a tube amp? Pretend it is a 380 volt supply rather than a 38 volt supply, if it helps.
Are those resistors getting hot? That would point to a shorted CR56 zener, or a grounded B+.
Is this thing blowing fuses? Did you check the two large rectifiers ther for open?
CR52,53 serve that B+. Check them both. You have the AC right at their legs, right? Don't assume because it is elsewhere that it actually gets to the diodes. Flip the board over and look for solder failures in the area of these parts.
How about this scenario. R172 is OK and measures the 98 ohms or whatever, but the end of it at the test point either the solder has failed so it is not connected, OR the wire leg is broken off and the part is held in place by glue, with the leg remnant not touching.