Sierra Towers do not come with bi-wire / bi-amp inputs. We do offer this for a $75 per speaker fee, but to be honest - there is no reason to spend the additional money. Passive bi-amping or bi-wiring offers no performance advantage.
Fantastic match with your NAD electronics. I have been using using NAD electronics since the late 80's
Best to give us a call or send us an email regarding estimated return shipping fees - but I wouldn't be concerned about it. We sell a lot of towers and average only 1-2 pair returned per year. We design these to be the last set of speakers you will ever want