well ive decided on the 12w7.
Emailed PG about the impendance and got this reply:
You could wire the sub parallel .75ohms you are correct, but I do not suggest it. The amp will work but get very warm and draw large amounts of current. The life of the amp will most likely be decreased as well.
Thank you,
Errin Keeling
The amp will be running along with another TI amp powering components in an E46 trunk which is annoyingly well sealed which means it gets hot in there as it is so think 12w7 will be the better option.
Thanks for the help neways guys.