I have a pair of Nd 6.1s in my sister's car and no matter how I amp them, I can't get them to sound good. Granted I may have a poopy set but I dunno... they just seem to break up beyond a certain (very low) amount of power. Not at all the sound quality I associate their subs with. Their subs are a different animal... absolutely no out-of-band breakup that I can hear... results in absolutely perfect up front bass, even with high crossover points (ran a set at 80 Hz/12 dB and could hear no bass coming from the back at all... everything was totally focused in front of my face)
When I collect the time and money I plan on dumping my two CDT MS-80s in corner-mounted enclosures and going with a single Nd12 in the spare tire well.