Impedance????

First question: impedance of what ? There are lots of impedances through the chain of a hifi.

Second question: Why ? What problem are you trying to solve ?

Jim,

If you split a preamp output into two poweramp inputs then the preamp sees the poweramp load in parallel.

When two equal impedances are put in parallel the resulting impedance is halved.

So running a "Y" type splitter from a preamp to two poweramps will reduce the input impedance seen by the preamp to one half of the input impedance of a single poweramp.

For example if you have two identical poweramps with an input impedance of 40k Ohms, and you connect both of them via a Y type split interconnect to a single preamp output then that preamp output will effectively see a poweramp input impedance of 20k ohms.

I think you're worrying too much.

You don't have to match the preamp and amp impedance, in fact quite the opposite.

The output impedance of the preamp has to be low (few hundred ohms or less), and the input impedance of the amp has to be high (10k ohms or more).

It's really only a big factor if you have long interconnects with high resistance or high capacitance, so that cable losses are significant. Under 1 meter it will make little difference.

And, in any case I expect the preamp will have multiple outputs: they usually do.

Post back here if it doesn't sound right. If the input impedance of the two power amps is too low it will likely sound lacking in dynamics, and a bit rolled off in the high frequencies. If it sounds dynamic and sparkly all is well.

Jim, I highly, highly doubt it could ever damage the amps: there's just not enough power coming out of a preamp to do damage. Perhaps a really terribly designed preamp might fail.

To comment on whether the sound might change I'd have to know the output impedance of the pre, the input impedance of a single power amp, and the capacitance and resistance of the interconnect.

In most cases, with an active preamp, I highly doubt it would affect the sound. With a passive preamp, and/or power amps with input impedances under 10k ohms it might lose dynamics and roll off the high frequencies, but really you'd have to try it.