Noise and other peculiarities of NuForce amplifiers

Why does NuForce have low audible noise when the amplifier is idling (no input signal)?

NuForce amplifier has SNR (Signal To Noise Ratio) of 120db (at 100W) so it is exceptionally quiet when playing music.  NuForce's feedback and control system runs at 1MHz and actively performs noise cancellation in real time (comparing noise with music signal) at every switching cycle.  But when there is no input signal, it merely amplifies noise in the system and therefore you can hear hissing noise from up to 1 ft away from the speaker. When the source device (Preamp or CD player) is turned off, it acts as a noise receiver/transmitter and could generate even more noise for NuForce amplifier during idling time.

Why some NuForce amplifiers have more noise than others? Why are there variations from unit to unit?

Switching amplifier uses hundreds of components in its circuit and their values have variations so it is expected that some units have slightly more idling noise than others.

Why is there a small DC offset between the two +/- speaker terminals?

The op amp has variations and will result in 0.05 to 0.14 volts on the speaker terminals. 0.14V is equivalent to 2mW at 8 ohm load.  This is hardly a concern for your speakers.