Continuing my notes from 3 posts above.
Wet/Dry pot doesn't work for ANY AU plugin.
For example, it works on CamelCrusher VST, but not for CamelCrusher AU.
http://www.camelaudio.com/demos.php#demo_id30
On most AU's the audio is muted when pot at 0%. On other AU's the effect is 100% on even though the pot is at 0%. These also have the phasing issue when at 50%. Sounds like PDC'd and un-PDC'd signals are combined.