CF, lets try: 1. The phase switch at 0 has the sub cone pushing out and pulling in at the same instant as the main speakers; at 180 it's in when the mains are out. The 180 might help in some situations due to a difference in room location between the sub and mains, but if you notice no difference leave it at 0.
2. I'm not familiar with there being such a switch on the EP125 and this may be something relatively recent that others can comment on, but I'll take a guess. There's a belief(misguided in my opinion)in some quarters that a sub should have more "oomph" for movies than music, so a switch is provided that gives an artificial boost to mid-bass frequencies. If this is the case, I'd leave the switch at "audio".
3. The size of the speakers has nothing to do with the size of the speaker wire. The primary factor is the length of the run from the receiver to the speakers. Unless you have runs longer than, say, 30 feet, regular 16 gauge lamp cord is fully adequate.
4. I'm not familiar with the 5200 and I'm not sure what +2 represents, but unless it's more than about 3/4ths up, it isn't anything unusual that needs to be changed.
