I then soloed track 1 and inserted an eq with 12dB of boost at 1K. This shows as a hump on the 2 bus inspector - again as expected.
Undoing the solo, and adding similar eq to the remaining tracks makes the overall signal bigger, but the hump stays the same relative to the broadband signal level. Drop the level on the 2 bus and you get back to pretty well where you started (given that we are dealing with broadband noise here) which means that in the mix you can not tell if you have one track with a 12dB hump or 24 of them - which suggests to my feeble thinker that the idea of preamp tone stacking is misleading - you may be MISSING other colours, but I don't think that you are "stacking" anything; which makes a sort of sense as the tones are adding in parallel rather than in series - you are effectively multiplying by 24 then dividing by 24.
If, on the otherhand you add the eq multiple times to a single track then the change really does stack, the changes become cumulative and change the shape of the signal. Obviously this looks at amplitude variables only and takes no account of any other factors such as phase.
Food for thought or barking-up the wrong bush?