I'm looking at two decks right now, they have the same voltage for their preouts but different numbers for ohms after it. One is 1k ohm and the other is 10k ohm.

How does this make a difference in the end to me?

 

The higher the ratio of the input impedance of an amplifier (or processor) to the output impedance of a deck, the higher the noise rejection.

So for a given amp, the deck with the 1kohm output impedance has 10x better noise rejection than the one with 10kohm.

Now, toss in the fact that amps don't all have the same input impedance and you can get situations where the amp and deck are too close and that monumentally increases your chances of noise. Then if you have to split your preouts to a couple of amps you can almost bet on noise.

It seems to be less of a problem these days but back in the day we always tried to avoid running certain deck/amp combos.