"Also, exactly how do you “short” the inputs, can this potentially damage anything, and what effect are you expecting?"

There are "shorting plugs" made (probably easier to make your own if they are not available locally). Perhaps radio shack has some, if not, purchase some RCA plugs (the kind that are on audio patch cords of the type that would be used to connect a tape deck to a preamp or (for single-ended use) a preamp to a power amp.

To make your own, solder a wire from the center conductor to the outer shield. Shorting jacks are sometimes used to lower the noise generated by an unused (open) input on a piece of audio equipment. Except when testing/experimenting, they are not needed and it sounds as though they are not needed in your case.

Shorting plugs were much more common back when pre-amps had really high gain inputs for tape heads and such and when the electronics were much more noisy than they are today.

As long as you are connecting shorting plugs to the INPUTS of audio equipment, you should be OK: no damage should result. Connecting shorting jacks to the OUTPUTS is a different story and should be avoided if you like your equipment.

Speakers with a 90dB sensitivity (usually rated at 2.828v [1 watt into 8 ohms] at 1 meter ... yada ..yada) is what I would call average sensitivity and should not "highlight" whatever noise is generated by the preamp and poweramp, provided the noise levels generated are reasonably low.

Paul