Did you find any solutions that allowed you to use itunes with your 5.1 setup where you got all speakers working?
Yes.... but you might not like my solution. My speaker system has a controller that sits on the desk, it has a headphone socket, plus controls for the amp; one of the buttons lets you switch between stereo and 5.1. I use that, sorry.
P.S. couple of points I forgot to mention. In the Aggregate device editor you can drag the sound outs up and down, this means that, for example, you can have FL&FR on streams 1&2 -I do not know if it has any impact beyond aesthetics. Make sure all channels are set to same bit & sample rates. You can make ad-hoc multichannel using a couple of $5 USB sound dongles, make sure you have the clock set to build-in audio to avoid channels going out of sync. Oh, you can rename your device by clicking on it.
The most powerful function of this tool is the ability to create an aggregate INPUT device. (e.g. join up line-in and mic-in and you have a 4 channel multi track recorder) You can then select your new aggregate device as a sound source in Logic or GarageBand. (Works w/ protools & Cubase too). Want to add more channels - you are only limited by the number of your firewire and USB ports. (Well.... I made a monster 24 channel beast on my old powerbook; recording 24 tracks chocked the laptop HD promptly (or the CPU, it was a 550MHz G4).
This has to be one of the least documented, most powerful tools that we inherited from CoreAudio - why does Apple keep so quiet about it always puzzled me. The interface looks like it came out of an undergrad Java 101 assignment, it's almost as if a programmer left it there as an after thought.
Oh, and the device for logic has to have at least one out... At least it used to have to.