You might want to look at Steven Slate Digital VCC and similar products.
It is an attempt to emulate the sonics (distortions, phase shifts, etc.) of consoles (Neve, API, SSL, Trident) as an ITB replacement for a summing box, and, of course, the real thing.
I've just purchased the VCC -- to me it has just the subtle glue I'm looking for.
As usual YMMV.