in the old days there were no PA systems as we have today and fender amps were used for vocals as well. in fact. iirc, that is why there is mixing resistors at the inputs so multiple instruments or mics could be plugged into them.
that box is more like a fenders NORMAL channel, like the alembic F2B, but without a mid control like the normal channel of a deluxe reverb etc..
to be more akin to a jerry pre that third gain stage really excites things, which is missing. reverb is also a must for jerry.
"The process of selecting the tone on the guitar is an aesthetic process like any other, so you try a lot of different things. Most of the things I’ve tried I’ve tried in live performances" ~ Jerry Garcia