Bypass sending a full range signal to the mains even when they are set to small, and no sub output whatsoever would just make me happier than a clam

To me, you have a 2 channel source coming in. When in bypass, don't mess with it. Don't apply any type of filter. Don't duplicate it and route a signal to the sub. Don't do anything with it. Just send the most direct signal possible straight from the 2 channel input, to the main's output.