Interesting to hear experienced folks say 1 speaker doesn't matter. I swear I heard a difference in my Princeton (single 12" JBL). I use the battery test: hooking up a 9V battery to the speaker terminals (and removing), the positive side of the battery connective to the positive terminal of speaker will flex the speaker out towards the front of cabinet. It will stay flexed until you remove the battery.
I don't doubt that technically it doesn't matter with only 1 speaker, or that there are many more factors which come into play. I also don't doubt that the single speaker seemed to produce less volume wired with + from amp to - on the speaker. Maybe one of my terminals is just dirty or it is in my head. "Feelings" often bear little relationship to objective reality...but I hook up + to +, even if merely to make me feel better.
If the JBL has been reconed it may no longer be BLACK = positive. I think the newer JBL's have switched to the industry standard RED=positive; not sure if the E120's ever had this switch though.