Thanks for clarifying that, although I'm still somewhat confused. Let me recap to see whether I understood you correctly. If I have a two speaker cab I obviously only use one speaker output. I switch that output selector to 16 ohm and install 2 G12-65 8ohm speakers wired in series. Correct?
Yes
What difference in tone would I get if I had the output selector at 8 ohm and had 2 G12-65 16ohm speakers wired in parallel? Or any other combination, e.g. output selector at 4 ohm as in Fender amps? Quite frankly, all this ohm business has always been a book of seven seals to me.
I'm not sure you will notice much difference but it has been mentioned in here that the amp sounds best if you are using all the OT windings ie 160hm
Furthermore, there are also two output sockets in my Ceriatone JTM-45 BB. Are these also wired in parallel?
I'm not familiar with the BB, you have to check the layout, but I would not expect it to be a series connection.