Shorting plugs....

Seems like you got your answer, but I can confirm that covering the inputs and outputs makes a significant difference to the noise floor of my system. I'll have to test the comment about only RCA needing covers as I'm currently using covers for both RCA and XLR. As mentioned above, never use shorting plugs on outputs.

Concur with Chadlinz, it's an impractical "solution" and just because "the improvement is material" *in your system* does not mean that it will be in anyone else's system.

Also, when you indicate that you turned off components, what does that mean, did you unplug the components, turn off power at the wall or simply press the power off switch on the components?

It's interesting the lengths you are going to to avoid the question. Your statement was that simply turning off components did not yield the "material" improvements that unplugging them from the preamp does.

Again, what do you mean precisely when you refer to turning off a component?

That's not turning it off if it's a unit with a switching power supply (i.e., most consumer electronics these days); the only way to turn off such units is at the wall or unplugging them.

In any case, what was the configuration you used to test on since your current system can't accept more than one input? In addition, how close were the electronics to the system?