I too still have my eighth nerve modded ZR1600 and just love it. Great bass control and open, clean extended highs, at least in my opinion anyway.
I actually had and opportunity to compare the amp to the NuForce Ref 8 monos in my system. Well, it certainly wasn't obvious to me that the ref 8's were superior to the 1600, which I kinda expected given all the hype about them. Sure, they were darn good, with a laid back sound, decent bass and clean highs. But I gotta say that the NuForce were a little too laid back for my tastes. I thought the 1600 gave better bass control and just a more full presentation across the spectrum. Now, I use a passive pre-amp (sonic euphoria) so that may have a lot to do with it. I believe the 1600 has a 100K input impedence so it is easier to drive that amp. Anyway, just thought I would chime in about the 1600. A great amp and I am even thinking about having it fully modded by RAM or Emperical Audio sometime down the road if I can come up with the cash. There have been some positive posts about those mods, taking an already great amp to another level. Peace