I guess you wouldnt need to switch the leads again at the amp...as they have already been switched (for JBL polarity) at the jack on the cabinet. If you dont get it right the 1st time just switch them around...the speakers should jump outwards at first when a signal is reproduced. This can be kinda tricky to spot though since it immediately bounces back in the opposite direction, hence the battery test.
I do not have a Mac unfortunately so any peculiarities of them I will be unaware of.
Smolder, I am just referring to a single speaker not parrallel/series wiring (although the same principle would apply). Basically I am talking about wiring the speaker so that it pushes air forward INTO the room, rather than backwards into the cabinet etc. I have heard folks say it doesnt matter for a single speaker though (or more if they are same direction), could be, but I thought I noticed a fairly significant difference with my D123 wired backwards, and it does no harm even if it doesnt matter. If it is pushing air to the rear of the speaker in creating sound, the cone would be inverted practically speaking as well. Of course it still produces sound if at a lower volume depending on the cabinet/location. With 2 speakers I make sure they both push air forward in unison.