Hey, I hate to be the bearer of bad news, but:
I tried a bunch of different speakers as rear speakers. Only one of them (a sony speaker) was even close to matching the volume of the front speakers. And it was only about half the volume.
You can set the fader way, way, way to the rear to equalize the sound, but that just does not seem right.
I really wanted to utilize the extra power/outputs of the OEM amp by using it for rear speakers, but finally bit the bullet and added an amp for rear speakers. That has turned out great. Now my rear speakers can be adjusted to be equal or even louder than the fronts.
The front stock speakers (maybe because of their enclosure) are ridiculously efficient. It is probably impossible to find some rear speakers that will match, volume wise. At least it was for me.
On the bright side, my rear amp and speakers sound better than the fronts. The bass is sharper and the treble seems to have more life to it. I am using the OEM amp (rear speaker output) as an input to the new amp. Works for me, but did not for others.
Albuquerque, NM