Do you have active crossover control on your headunit? or only on your amp?
What are your rear speakers, model and power handling.
As you probably know by now, everyone's ears suit a different taste, what you like will most likely be a personal thing. Get fiddling! And adjust every few days when you pick a flaw!

As a guide, sub-bass goes up to about 80odd hz. You dont want this from small rear speakers, doubly so if your using a sub.
So, if your not using a sub, try a HPF at ~100hz, 12db/oct. If you are using a sub, bump it up to say ~120hz, with a steeper slope.
Then again, if all you have is amp crossover control, play between 80-120hz.