Wow, I had to be away from things for a day, and the discussion really heated up!
I'm very glad to see HPF is back on the table.
Personally, I'd really vote for a 6db slope -- 12 or 18 seems more like what you want for ENG, not music taping. Something like 80-100hz is probably ideal. 80hz = 3db down, 9db at 40hz, and 15db down at 20hz. That doesn't seem wimpy to me, but 12db could work ok too, though in that case I'd vote for a 50-60hz knee. Of course, if some internal jumpers like the Grace V3 are possible for choosing 6db or 12db, that would probably be great, though it may well be asking too much in terms of versatility vs cost.
One extra switch available? I think it'd be totally cool if you could have a switch to choose to run the preamp with the transformer inputs sometimes and run the preamp with no transformers and capacitor coupling the other times -- make it like two distinct preamp flavors in one!
As usual, keep up all the great work and thanks for keeping us as part of your process.