So, I have a shot at adding 1 nice .1uF Silver Oil cap, I don't have schematic, wondering thoughts on which one to bypass with it. I was thinking the 1.8uF, what do you think?
The whole thing leads up to the tweeter, certainly the 4uF would be easiest from an access point of view. Wire to the left is input, wire (obscured in pic) to the right leads up to the horn tweeter.
You should put the boards out of the box permanently. That move resolves most problems ( well, one...) and gives infinitely more benefits than to put a cap in parallel.