I agree with what the others have said. Even with the exact same amps, my experience is that 3 out of 4 cases the benefits aren't worth the effort and cost of 2 amps - you're better off trading in your NAD for a better amp.
I'm doing vertical passive biamp on one of my systems and it does perform better than other single amp configurations, but the speakers are unique in that they take advantage of it. I've tried other speakers and amps with near zero resultant benefit. I also tried horizontal biamping using tubes on the top, ss on bottom - it was simply too much effort and trouble for the benefit. My speakers sounded better using the tubes full range.
I'm doing vertical passive biamp on one of my systems and it does perform better than other single amp configurations, but the speakers are unique in that they take advantage of it. I've tried other speakers and amps with near zero resultant benefit. I also tried horizontal biamping using tubes on the top, ss on bottom - it was simply too much effort and trouble for the benefit. My speakers sounded better using the tubes full range.