Ron in AZ said:
rs - thanks for the comment. I think my best course of action is to wait (and wait....) for the D7 mark ii and keep an eye out for a used 100-400L!
Unless Canon has some amazing new technology up its sleeve, it seems highly unlikely that a 7DII, if released this year, would have significantly better image quality than your 7D; so that if you want better image quality from the lenses you have, especially in low light, it would make much more sense to me to get a 6D (unless you're frequently tracking fast moving things) - leaving aside the reach issue, your ff lenses will all take better photos on a ff body than they will on a crop body, and you may well get better results by cropping on a 6D to make up for the reach you're losing compared to the 7D (i.e., you will do the cropping manually rather than letting a crop-sensor do it for you automatically whether you like it or not).
As for the 100-400, its image quality isn't quite as good as the 70-300L, and the difference between 300 and 400 isn't that much - if you want significantly more reach, regardless of what body you get, it may make more sense to get the new Tamron (unless someone shows that the Tamron at 600 isn't any better than the 70-300 cropped, which could be the case, I suppose). Or if you want lots more reach, consider the Canon 50x superzoom point-and-shoot camera, which goes twice as far as the Tamron lens for half the money or less (see posts on this forum extolling its virtues in some contexts). Or consider adding a micro 4/3 body and a Panasonic 100-300mm lens (equiv. 200-600mm) - remarkably good image quality, and small and very light compared to the other equipment you're considering.
As you don't seem to be in any particular hurry, it might make more sense to, say, rent a 6D and see whether you agree that the ff lenses you have perform significantly better on it, and appreciate its superiority in low light (hard to imagine that you wouldn't, but who knows?).