I have found that the AUMatrixReverb and AUVectorPanner both work if the buffer size is 1184 or greater. If it’s 1152 or less then they both report EXC_BAD_ACCESS from within processBlock (AUMultibandCompressorEntry)
These tests were run with v1.53.40 in the Plugin Host Example. The Mac “AU Lab” app is able to load both AU’s at low buffer sizes on my system.
13" Macbook Pro w/2.4ghz Core 2 duo 4gb ram.