5150/6505 all the same, no difference between "block letter" and "signature" other than the decorations on the cabinet.
Was there some confusion over the number of relays?
I doubt you have a voltage issue. If your post control won;t turn it down, then most likely either the control is open or it has lost the ground connection at its low end. Does the control adjust the volume at all? When you are on the ultra channel, the post doesn't work right, right? But does the clean post control have any effect on the ultra level? And for that matter, when you are on clean, does the ultra post affect the volume in any way?
Check the control itself for opens or loss of ground.