I think that your question may have already been answered by that mythical "Turbo" being that some of us see quoted here but never witness in person... If your client can set the asymmetry control to 100 (ie, 100%+/-) and see what the mod monitor thinks then.
As you mentioned, the processor should always be at the transmitter end for max peak control. The STL could definitely be messing with your symmetry. Especially if it is being over-driven itself. The quality of your STL shot can also impact mod levels and symmetry...