ZD, he's using a Lightspeed, which attenuates via light-controlled resistors. When he turns off the preamp, power is removed from the LEDs which control the resistors.
Bruce (B_limo), what I would suggest is that you verify that no significant noise is heard for all settings of the Lightspeed's volume control when the Lightspeed and the source component are on but no audio signal is present. If that is the case, I don't see that there is a problem.
It sounds like the noise, whether it is being generated by the input circuit of the amp for that channel, or is being picked up by the interface cable, or both, is eliminated when a relatively low impedance is connected to the amp input (relatively low with respect to what it may be when the Lightspeed is turned off). That's not surprising. Everything else being equal, higher impedance circuit points are more susceptible to noise issues than lower impedance circuit points.
Since the proper turn-on/turn-off sequence for any system is amp on last and off first (to prevent "transients" that may be generated at turn-on or turn-off by the preamp or other upstream components from being amplified and applied to the speakers), I don't see that there is a problem.