Matching preamp to amp

The simpliest way is to take preamp of the same brand as amp.

Other cases you should match the input sencitivity of amplifier to the gain of preamp:
If input sencitivity of amplifier is low than preamp should have a high gain or the other way arround...

Not all cases are trivial and you should also know the other variable such as input impedance. In trivial case the input impedance of amplifier should be 10x of output impedance of preamp but this ratio is valid if the output of preamp is relatively high since transfer of voltage has substantial losses and the losses are certainly not linear since you're dealing with complexed signal i.e. music.

The ideal case is when input amp's impedance is too high and the output impedance is too low but if the output voltage capabilities are substantially higher than input sencitivity of amplifier than even 10x-rule has its own tolerances in that case...

Marakanetz has it right.

Usually the easiest way is to use the same brand, but that may not necessarily be what you'll musically like the best. Did that make sense? Heheh.

Components within a certain brand shouldn't have impedance matching problems, but when mixing different brands careful selection is necessary. Case in point. I auditioned a friend's Pass Aleph 3 for a month and loved its sound. Fabulous on guitars and great layering front to back! But its low 23Kohm input impedance wasn't a good match for my system, which resulted in big loss of dynamics even with the volume cranked way up. In the end, even with the great sound I couldn't consider owning it with my current system.

What Marakantz has detailed is a good guideline for matching any preamp to an amp, but it becomes even trickier when throwing a passive preamp into the mix. Then you have to consider the output impedance and output voltage of your source components along with the amp's input impedance.

Thanks form the info Marakanetz and Gunbei! Being new at this I didn't know it was that complicated. I'll do a lot more investigating before I decide on a pre amp.