I wonder if Dilley was the pioneer for using a virtual ground summing amp in a pro desk? His patent was filed in 1969.
As usual, in these patent files, it's very difficult to find out exactly what is new and what is just current state-of-the-art, but it seems it is more about the arrangement of transistors than the virtual earth summing.
In fact, it is often easier to patent something that does something than a conceptual notion such as vitual mixing.
I think the concept of mathematic summing with operational amps was around for some time, with the current-then interest in analog computers. Audio guys may have just seized the opportunity.
If you read carefully the text, you may notice a curious claim that "Q2 and Q3 are connected to operate in common-base mode"; although it is true for Q3, which receives signal on its emitter, it is not the case for Q2, which, being part of a diff pair, receives signal on its base and outputs on both its emitter and collector.