Vp = 462, Vs = 454V, Vg = -39v and 35 & 36mA looks good.
I d´ont know how to interpret the measures in AC that you put but associating it with what happens I have remembered that the output transformer in that amplifier is somehow weak. I found it broken in two units.
Although it is not decisive you could measure the resistance between the primary to at least check for symmetry. With dynamic signals at the input a series ammeter on the plates can also show an elevation of intensity that is not matched with the developed power.
If you have an accessible output transformer, you could connect it eventually to see what happens.