I have successfully routed my system audio through AU Lab via Soundflower, but it clicks.
To my knowledge, that clicking sound usually means that buffer sizes are out of wack (either too small, or not consistent across any applications in the signal chain).
Yet, trying to match the settings as follows doesn't seem to fix the problem.
• In Soundflowerbed, buffer size = 2048
• In Info pane for AU Lab (CMD+i), selected 'Open in 32-bit mode'
• In AU Lab preferences, buffer size of all devices = 2048
• In System Prefs > Audio Midi Setup, Format of Built-in Output = 44.1k, 32 bits, to match Soundflower.
A few observations:
• In AU Lab, regardless of what settings I've selected in Preferences, it shows the Frames settings of Built-in Output to be 512, which is not necessarily what I have it set to. That figure gets updated for Soundflower. Not sure what that's about.
• Setting all buffer sizes to 512 to match the above does not fix the problem either.
• Clicking is present with all audio, consistently, and begins as soon as audio starts. If there is no audio, there is no clicking.
• In Audio Midi Setup, Soundflower says its audio is of the 32-bit integer variety, while the closest option Built-in Output has is 32-bit float. I don't know enough about digital signal processing to know if that's an issue.
I am on the Mid-2014 model MBP with OSX 9.4, Soundflower 1.6.6, and AU Lab 2.3.