Your heaters have 6.3v if they light up and it amplifies. If you really had only 3v, it wouldn't work. You are taking readings to ground, which is either side of the center tapped winding. Only thing that matters is that you have 6v ACROSS the tube heater, NOT to ground.
The tube DC voltages do not bother me at all, really. This is just a guitar amp, nothing precision about it.
As to power supply, the 340v is on C25A, not C25C. And I think you have the other two out of order. I see no way for C25C to be higher than C25B. In any case, I see no problems. The fact they are out of order concerns me that you may have wired them in the wrong positions. The A, B, C sections are in decreasing capacitance and decreasing circuit voltage, in that order.
No. Power transformer failure is THE single least likely failure in any amp.Could this be a Power Transformer issue?