... As I continue on, I actuality start to lose volume. ... Also, on both channels the opposite volume control is acting as a presence control for the opposite channel.
Willabe already explained this. There's an 85% chance your amp has nothing "wrong" with it.
The Gibson GA-20
volume controls are wired like a Fender 5E3 Deluxe
. They are backwards from "proper" volume control wiring, and work by loading down the signal.
But if you do not have the unused channel's volume all the way off, as you turn up the volume for the channel you are using, your sginal partially bleeds back through the other volume control. This tends to change the tone of the channel you're using.
I have had 2 and currently have 1 GA20 and both have had a 6SC7 in the inverter position factory stock.
This very well could be stock. But as shown in the schematic I linked, at least one version had a 6SL7 phase inverter. But Gibson is notorious for changes without updated schematics, and for amps with the same model number and wildly different circuits.
Look at the data sheets for the 6SC7
. They are largely the same tube, but the 6SC7 has a shared cathode and different pinout.
The quickest deciding factor is what pin 1 is connected to. If no connection, or it's connected to ground, it's a 6SC7. If pin 1 is being used for a grid connection for the phase inverter (compare to what else connects to a phase inverter grid on the schematic), then it's a 6SL7.
My guess is if the amp works at all, you probably have the right tube in the socket.