A single gain stage could be shared between two channels, but you'll have to consider exactly what you want your amp to be, and whether such a shared stage makes sense.
You have 4 preamp tubes available (8 triode sections). Assume that one of those will be your long-tail phase inverter, which takes up 1 whole tube/2 triode sections.
So you have 3 tubes left.
Ignore the Bright channel input on the
5F6-A Bassman, and look at the remaining triodes used. You have a Normal channel input stage, another gain stage, then a cathode follower. Afterwards is the phase inverter, which you already have accounted for, so you'll need 3 triodes to replicate one channel of the 5F6-A Bassman.
That leaves 3 triodes. If you look at the
AB763 Super Reverb, you'll notice the Normal channel has only 2 triodes before the phase inverter. You could copy that, and be left with 1 triode not doing anything.
So you don't have enough left over to add reverb (without punching another hole and adding a tube or 2), and adding trem with only 1 triode might be difficult to integrate well into the existing circuit.
A different option might be to use the leftover triode in the AB763 channel to mimic the extra gain stage found in the Reverb channel of the Super Reverb (and other AB763 amps with reverb). The catch is the dry signal of the reverb channel gets little extra gain boost from that extra triode, as it mostly boosts the reverb signal in that channel and helps make up some of the loss from the mix network used to combine dry and reverb signals.
To the Super Reverb's Vibrato channel dry signal, it looks like there is a voltage divider between the 2nd and 3rd gain stages made of a 3.3MΩ resistor and a 220kΩ resistor, dropping signal strength about 94%. It's arguable that the gain which then follows barely does more than return the dry signal back to its original strength.
So where does that leave you?
Knocking a signal down just to boost it back where it started doesn't do anything but add noise (if you're not also getting some other benefit like reverb). Adding a gain stage that isn't present in the original AB763 circuit takes you a bit away from the original sound. And I can tell you from my experience in this situation that just using 2 triodes and the phase inverter (like the Super Reverb's Normal channel) has a blackface-type sound different from the tweed 5F6-A sound in the other channel.
So you
could just leave that triode unused. Or you could choose not to be exact to the original circuit and use the extra triode to either parallel an existing gain stage, or to cascade (with ot without a voltage divider ahead of the added stage) the existing preamp into this gain stage and go after a different sound.
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