That sweet sweet blackface treble sound

[22uf]

What is it? Can it really just be the speakers?
To my ears it sounds like there's a high pass filter going on that is compressing the higher frequencies?
The ceramic speaker thunk and plunk aside, where is that filter happening in the AB763 circuit?
The phase inverter coupling cap maybe?
I have got within the ballpark now with a 75 AB763 conversion I'm working on (one of a long line hehe). The GA-SC64 speakers I picked up really do get close.
I've added 10k to the grid stoppers on the inputs for V1 & 2 that has really thickened it up nicely. All that is missing is that shimmery top end. 
I just can't seem to get that sweet compression in the high frequencies.. 

[HotBluePlates]

I'm not sure those are the right assumptions, nor that you're asking the right questions.

I don't know what all is in your "conversion" but I found that old reverb tanks have the rolled-off treble while modern tanks are brighter/harsher (to my ear).  If you're playing through the Vibrato channel, the tank's effect on the sound matters for the overall EQ unless you're playing bone-dry.  The reverb's lack of definition (or "blur") can take the edge off the attack.

     However, an AB763 Fender does not compress the way an early-60s Reverberocket does.

This is a 1966 Pro Reverb with an original reverb tank:

https://soundcloud.com/hotblueplates/next-time-im-in-town-1

[uki]

Some extra characters on the HBP's link, not working.

Here:
https://soundcloud.com/hotblueplates/next-time-im-in-town-1