If the whole upper reed bar is quieter than the bass reed bar, make sure that the treble pickups are in fact transmitting to the amp. It is conceivable that you are picking up the resonance of those notes through the body into the bass reedbar. I doubt it, but please make sure: disconnect one of the wires connecting the reed bars in the middle, and see if the problem is worse or identical.
What's the serial number of that instrument? I'd love that, and any dates you find inside, for my research:
https://www.Drwurly.comYou can send me a private message here.
The 140A amp never has the death cap, as far as I know. Of the 60's models, only the 145/720 tube amp (including any A's or B's) does.
The 140A is a completely different solid state amp from the 140B amp. You want to look at the schematic for the "140 amp." (not "140B amp") The 140A amp is a minor revision of the 140 amp--so minor that they didn't bother to make a unique schematic for its updates. It's the same board, with just a few values changed in some caps and resistors, and a couple (crucial) resistors added.
The famous problem with the run of 140A's and 145A's is that there was a bad batch of reed screw washers partway through the run, and those instruments are prone to washers cracking. Not all 140A's have this problem; seems to be the later ones.
Even the instruments without the washer problems are hard to tune. They just aren't easy reed screws. But it can be done! I've done it.