Actually, if you're going to go all in on a stereo modular mixer, Toppobrillo's Stereomix 2 would be a very good place to start. Sure, it won't fit in the current cab, but it DOES give you CV over panning, level per channel, a cue mix (very useful for adjustments on the fly), mutes per channel, AUX sends (with CV over level per channel) and a stereo AUX return, and so on. And while it's not cheap at $479, it covers ALL of your mix processing/control needs. And then some.