In my system, the D-100s were more "tube-like" in the midrange than the Nuforces. Being a long time tube amp lover, this was very important to me. In addition, I felt the D-100s had a tad more width at the back of the soundstage.
Due to the fact that NuForce has nearly 0 phase shift at 20Khz (compared to 15-degree or so phase shift in Hypex UCD module or Icepower), the speakers should be slightly toed-out to really focus with NuForce Ref-8. Read the latest Soundstage and Stereotimes reviews on NuForce....