You need two downloads, each of which is free:

1. Soundflower. Download Soundflower for Mac - Allows applications to pass audio to other applications (beta). MacUpdate.com

2. AU Lab: http://images.apple.com/itunes/mastered-for-itunes/docs/au_lab.zip

Install, reboot. For use with iTunes, you need to select Soundflower 2.0 as the (virtual) output device.

Open AU Lab, and set the input device to Soundflower 2.0, and the output device to what you really want (your dac, bridge, etc).

<p><a href="/monthly_2013_03/58cd9bc396f76_ScreenShot2013-03-25at1_22_01PM.png.90e01153a17bf9ab18a1c81f0478e152.png" class="ipsAttachLink ipsAttachLink_image"><img data-fileid="28156" src="/monthly_2013_03/58cd9bc396f76_ScreenShot2013-03-25at1_22_01PM.png.90e01153a17bf9ab18a1c81f0478e152.png" class="ipsImage ipsImage_thumbnailed" alt=""></a></p>

Click the "Create Document" button. Then select what AU interface you want. For example, this is the parametric equalizer:

<p><a href="/monthly_2013_03/58cd9bc3a406a_ScreenShot2013-03-25at1_20_24PM.png.87761bad6565dc301c264414baa11825.png" class="ipsAttachLink ipsAttachLink_image"><img data-fileid="28157" src="/monthly_2013_03/58cd9bc3a406a_ScreenShot2013-03-25at1_20_24PM.png.87761bad6565dc301c264414baa11825.png" class="ipsImage ipsImage_thumbnailed" alt=""></a></p>

Another potentially very useful option is the new Dirac AU (beta) plugin, which will allow you to use Dirac with everything:

[ATTACH=CONFIG]23217[/ATTACH]

[ATTACH=CONFIG]23219[/ATTACH]

[ATTACH=CONFIG]23218[/ATTACH]

[ATTACH=CONFIG]23220[/ATTACH]

You can save it if you want a permanent filter.

Voila!

You aren't restricted to Apple's plugins, either. Eg, the tube amp you always wanted as a room-heater:

<p><a href="/monthly_2013_03/58cd9bc3aa7eb_ScreenShot2013-03-25at1_32_28PM.png.3b70c7f0b989ca5032fcc80819bbb12e.png" class="ipsAttachLink ipsAttachLink_image"><img data-fileid="28158" src="/monthly_2013_03/58cd9bc3aa7eb_ScreenShot2013-03-25at1_32_28PM.png.3b70c7f0b989ca5032fcc80819bbb12e.png" class="ipsImage ipsImage_thumbnailed" alt=""></a></p><p><a href="/monthly_2013_03/58cd9bcb96a15_ScreenShot2013-03-25at1_22_01PM.png.cd2d635fbdda3c45e68afd29d3679934.png" class="ipsAttachLink ipsAttachLink_image"><img data-fileid="28406" src="/monthly_2013_03/58cd9bcb96a15_ScreenShot2013-03-25at1_22_01PM.png.cd2d635fbdda3c45e68afd29d3679934.png" class="ipsImage ipsImage_thumbnailed" alt=""></a></p><p><a href="/monthly_2013_03/58cd9bcba1248_ScreenShot2013-03-25at1_20_24PM.png.77269228024821422bda0e1814ec237d.png" class="ipsAttachLink ipsAttachLink_image"><img data-fileid="28407" src="/monthly_2013_03/58cd9bcba1248_ScreenShot2013-03-25at1_20_24PM.png.77269228024821422bda0e1814ec237d.png" class="ipsImage ipsImage_thumbnailed" alt=""></a></p><p><a href="/monthly_2013_03/58cd9bcba6d04_ScreenShot2013-03-25at1_32_28PM.png.7c046443bc96de068645a77d4cd814c4.png" class="ipsAttachLink ipsAttachLink_image"><img data-fileid="28408" src="/monthly_2013_03/58cd9bcba6d04_ScreenShot2013-03-25at1_32_28PM.png.7c046443bc96de068645a77d4cd814c4.png" class="ipsImage ipsImage_thumbnailed" alt=""></a></p>