I don't know why but GK seem to have a problem with contacts in the effect loop oxidising. You can work the jack with a bit of Deoxit to clean it up. Sounds like you got there already. I don't know how long the spray takes to evaporate, I wouldn't be playing it again that day anyway.
Another identical cab stacked on top will let you play to 300 with no PA. Single 15 will struggle if your guitars are way loud or your drummer is an animal. Usually the PA can help out guitar, not so much with bass.
If you're playing the kind of UK bars I was in back when I was on my OE, one 15 will do fine. You'll be getting more gigs by keeping the volume sane.
Team Trace Elliot #1, Mediocre Bassist #399, Old Basstard #86 Kala U-Bass #22
Swamp Kauri custom 5str. Stagg EUB. Krappy 5er FL.