I'll second Bill's comment. This excerpt from a Sound&Vision.com review seems to third it:
Impedance of the RX6 is a little low but not low enough to cause problems for any decent receiver or amp; minimum is 3.9 ohms at 150 Hz. The RX Centres impedance is a little more demanding, with a low of 3.0 ohms at 200 Hz. The RXFX is easy to drive, with a 4.8-ohm minimum impedance at 2.6 kHz, and a nominal impedance of 8 ohms. Sensitivity of all speakers is about average.
The NAD should be fine.
Marty
Impedance of the RX6 is a little low but not low enough to cause problems for any decent receiver or amp; minimum is 3.9 ohms at 150 Hz. The RX Centres impedance is a little more demanding, with a low of 3.0 ohms at 200 Hz. The RXFX is easy to drive, with a 4.8-ohm minimum impedance at 2.6 kHz, and a nominal impedance of 8 ohms. Sensitivity of all speakers is about average.
The NAD should be fine.
Marty