V=IR (Voltage = Current times Resistance).

P=VI or substitue the above equation and P=I2*R (sorry, can't superscript the 2, that's current squared).

The point of the math lesson is that power is directly proportional to resistance. If you tap into the rear speaker wires, you will change the resistance. If the powered amp has an effective resistance equal to the resistance of the rear speakers, then you will only get 1/2 of the power coming out of the rear speakers. Since it's a powered sub, you'll be able to recover the deep base but the midrange & tweet in the rear will still be lower than in the front. You can compensate for that with your fader but what you don't want to do is dork up the balance by only connecting to one rear speaker. You'll never be able to find a proper setting for balance & fader that will get the sound properly distributed.

All this theoretical minutia is happily provided by your tone deaf EE with severe tinitis. A kick arse stereo is completely wasted on me. As long as Brooks & Dunn comes in without a lot of static, I'm happy.