How do I send music over Firewire on an iMac?

Question

We have an iMac set up with a PreSonus digital PA mixer in our church.  Sending iTunes music over the stereo audio output from the iMac to the mixer is noisy.  We have a Firewire input on the mixer, and the input channel on the mixer has an ability to receive music and other content over Firewire.  We've tried just pluggin in the FireWire cable, but it doesn't seem to be able to send music that way.
How can I set up the output section of iTunes to music over FireWire?
thanks so much.

Answer 1

If your late 2009 IMac is the 21.5 inch screen model, the GPU cannot be changed at all as it is soldered to iMac's logic board.
If you have the 27 inch screen model, then Kappy is correct.
The GPU chip is a removable, replaceable socketed chip but it can only be replaced with a GPU chip that has the same socket configuration as the actual GPU socket the GPU chip sets in is NOT replaceable and is soldered to the IMac's GPU card.
There were only two GPU chips that can be used/accepted in a 27 inch screen, late 2009 IMac model, a 256 MB VRAM GPU and a 512 MB VRAM GPU.
You can only use GPU chips that were available for a particular IMac model year.
The 2009 512 MB VRAM GPU chip is very hard to find and very expensive to purchase and the installation should be done by Apple OR an Authorised Apple Service provider adding additional cost.
The total cost of replacing your iMac's GPU could be upwards of $500+ U.S.D.