I've got a 5 channel amp powering my Right, Left, and Center and the Rear Surrounds via the pre-amp output. I use the Onkyo to power the Rear Back Surrounds (or you could power the Front Wide or High but not more than 1 of these at a time). I'm using an Outlaw 755 for the other channels and the difference is noticeable between receiver power and 5 channel amp power.
I had an Outlaw 970, then tried the Emo UMC-1 (way too buggy), before settling on the Onkyo as my pre-amp. For the price including the networking feature, I couldn't find anything close to it out there with the same feature set.
PS - For anyone interested, I just picked up a AV RECEIVER/AMP/DVR cooling fan w/thermoswitch module off Ebay for my receiver after I got it back from ABL (HDMI board replacement). I have to say it does a great job of cooling off the receiver when it gets hot. I'm sure you could probably make one yourself for less than $50, but if you have no soldering ability like myself, its a pretty good deal and gives me some piece of mind that the receiver isn't cooking to an early death (again).