Solution #1 has the problem of the subwoofer's volume level being different between the coax and the line level...you might need to adjust the volume level every time you switch. Make sure to set the front speakers to LARGE as well. We don't recommend this approach.
Solution #3 looks good. Usually, this works well, as long as your main speakers are not too big. Since you are using bookshelves, the subwoofer will augment the bass as it rolls off, leaving a nice "natural" crossover between the sub and speakers. The HSU will tap into the bass routed to the left and right speakers. The surrounds generally don't have any bass, and the center doesn't have that much also, so you won't miss much.
One more thought...if your NAD has an extra PRE-OUT, you can implement a switch box system that I tried to describe here:
Basically, you plug in the PRE-OUT and SUB-OUT to the switch box, and switch between the signal the subwoofer will get. You probably don't have to switch the crossover in and out like my previous post describes. In your case, since the crossover will be about 80 Hz for the Ascends, you could leave the HSU's crossover to IN and it should work.
Let me know what happens