5G13 and the 6G13 Vibrasonic had a big OT. That in itself would separate it from the AB763 Vibroverb. he difference in rectification would yield a different map, too.....SS in Vibrasonic and tube in the AB763 VV.
Had I been a steel guitar player back then, I would have wanted the 5f8A high power tweed Twin and in the 6G period a 6G8 Twin.....with a 15" maybe. Double the power tubes = More power, right? Lower the input gain and Voila! Steel amp. IT doesn't look like Fender was overly concerned with the low note on the C neck, were they? They did not foresee the volume that would be wanted later on maybe. Just like SRV's VV amps and big strings, some strings and frequencies on a steel need/demand some adjustment to the input gain to maintain the notes' integrity.
I have never been offered a Vibrasonic...would be a treat to have one. I would imagine that original and exc cond value would be sort of hefty.