Magnepan MMG with ICEPower amplifiers

[JoeE SP9]

Dawnrazor:
If the mylar is on the front Magnepan has already latched onto that tweak. As for changing the plu and minus speaker connections; that's a question of absolute phase. You might want to try reversing the connections anyway. If you and your system is sensitive to absolute phase one way will sound better. It's possible you may hear better sound with the connections reversed. This applies to both speakers. You want them to be in phase with each other.
This means that connecting plus to plus from amp to speakers doesn't necessarily mean that you have preserved absolute phase. The number of stages in the entire amplification chain determine absolute phase. For instance some power amps have 3 stages, some 2 some have 4 or more. The number of stages in the preamp also count. Each stage reverses phase.
Preserving absolute phase is a much discussed and not always agreed on tweak. As with this and other free tweaks your ears and your system should be the judge.

krishna:
To find the first reflection points.
Sit in your listening position.
Have someone move a mirror along the side walls.
When you can see the speaker in the mirror you have found the first reflection point.
Apply sound dampening at that place on the wall, from the floor to just slightly higher than the speaker and at least as wide as the speaker.

This process also applies to the floor and ceiling. Usually there is a rug on the floor and significant others usually object to ceiling treatments.

From the description of your room you may have some difficulty getting an even left right spread. You have probably noticed this with your current speakers. Dipolar radiators (Magneplanars) will probably exhibit less of this room effect because of the figure eight radiation pattern.
The Cardas placement suggestions are a good place to start. However any dipolar speaker will need to be fine tuned for position. Let your ears be the judge.