Avid Technology PRO TOOLS MIX 51 - Manual (Page 17)

menu which shows the various Mix 51 outputs available (from each of the Main
Bus outputs and Send Bus outputs).
Panner or Mixer Not Getting any Signal
The track must not be set to “no output”, or else the Mix 51 plug-ins will not
receive any input signal (even if you donʼt want the main track to be routed
anywhere, you need to have it go to an unused bus or perhaps a separate stereo
output).
Canʼt Send to LFE Without Also Mixing into Main
Outputs
Use the Mix 51 LFE Send plug-in for this purpose – the LFE fader in the Mix 51
Surround Panner plug-in is post-fader.
Not able to set up a surround Reverb
Please see the Mix 51 demo session for an example of how to create a send bus
mix and bring it back into your main set of outputs.
Hearing Audio Glitches
You might be hitting the limits of how much RTAS processing can be done on
your system. Try increasing the HW Buffer size setting in the Pro Tools Playback
Engine dialog (note that this does increase the latency of processing in Mix 51).
Removing some other RTAS plug-ins could help as well (use the AudioSuite
version of many plug-ins to process the file for example, or record the processed
track and remove the RTAS plug-in to reduce the load on the CPU).