Yes, I understand that the Rig16 doesn’t have a separate DSP, but maybe it is possible to use part of the XMOS resources for simple equalizers on inputs? To control resources you can limit the number of channels (for example, activate channels where the equalizer will work).
I really enjoy mixing inside the Rig, where the computer resources are mainly used for sending and returning effects and adding a couple of virtual instruments. However, there is a significant lack of frequency adjustment options for external synthesizers.
