You can download the "complete" Gibson Service Manual here:
http://www.schematicheaven.com/manuals.htm
In fact there is no complete manual. There are 5000 variations it seems!
That amp does not match any of the printed schematics for EA50T. It has an interstage phase splitter/driver transformer like the GA15RVT (the brightest amp I ever heard).
Interestingly the power tubes are cathode bias, whereas most amps using an interstage trans like that feed a fixed bias through the secondary of the interstage trans to the grids of the power tubes. This one feeds the bias-vary trem signal through the secondary. That much is cool design.
Two things leap out at me
1) The interstage trans has DC flowing through the primary. I assume it is a 6C4 driver. To get good LF response in this arrangement, the transformer must be air gapped. I'm guessing that the transformer could be getting saturated with the 8-12 mils of DC and thus is light on the lows.
2) The tone circuit looks funky (can't read the values) but it looks like a variable midrange suck out design (i.e. a "Bridged T filter") with provisions for rolling off highs in a more traditional filter coming off of the top of the volume control.
This first tone circuit is basically the circuit that was in the Sprague couplates mentioned above, designed to put a big midrange scoop in the response.
For an interesting experiment, try jumpering this crazy thing out with a clip lead, going from the output of the coupling cap before the tone control into the resistor after the tone control. Or lift the ground of the cap in the tone circuit...that will also disable it.
There might still be room for some easy mods, but the fixes for the interstage trans circuit are relatively involved, aside from getting a beefier interstage trans designed for unbalanced DC in the primary.
It would help a little bit to replace all the old black cathode bypass electrolytics. I can provide a few for the experiment.
Hopefully, the new speaker will be good enough to make it enjoyable to play but there are a few other easy things to try. Send me the photo direct from the camera and I'll try to get the values off it --jroberts--at--io.com