im using a Marantz SR 5008 an I was thinking about adding a 5 channel power amp but I was told to make sure the pre-outs are balanced(XLRs)
bruin62, you were told wrong. You don't need balanced pre-outs. Most AVRs don't have them, and most people wouldn't use them if they did, at least not for a common Home Theater setup.
Your 5008 has 7 amps built in. So you can drive 7 channels with just your 5008. That could be the normal 5.1 plus the 2 Front Height channels. Or it could be a more traditional 7.1 w/ Rear Surrounds instead of Front Heights. Your call.
Now, about the amp. You don't need one, but most people here, in your position, would get one anyway. I did. My AVR is 7-channel like yours, and I got a 5-channel amp.
Instead of the speaker outputs on my AVR, I use the pre-outs for Left/Center/Right/Left Surround/Right Surround, and run them to the amp... then speaker wire from the amp to those speakers. I only use the AVR speaker outputs to run speaker wire directly to the Rear L/R Surrounds.
This means the AVR is only actually powering the Rear Surrounds. The amp is powering the other 5. Can I hear any difference? Probably not... but I am positive my system has more than adequate power for my speakers. It's kind of an Audioholic thing.
That's why you'll see people questioning you about whether you really need an amp or not. A perfectly acceptable answer is, "Probably not, but I
want one". That is the answer most of us here have used.