How to match preamp and power amp?

How to match them by looking at the preamp output impedance and the input impedance of the amp? Any magic numbers? I don't understand their relationship. Please help.
Buying within the same mfg. line takes all the guesswork out of equipment matching.They spend hundreds of thousands of dollars on R&D to insure their equipment works together.I'm not in any way saying mix and match does not work,just stating the obvious.
Ok, I am going to borrow from Wikipedia, and also add some things I have found to work.

If you think of your preamp as a source, and your power amp as a load, then you may consider that maximum power transfer can occur when the output impedance of the preamp matches the input impedance of the power amp. However, in high fidelity audio, it is typically considered optimum to have a source with low impedance connected to a load with high impedance. In that case, the power that can pass through the connection is limited by the higher impedance (so power transfer is not maximum), but the electrical voltage transfer is higher and less prone to corruption than if the impedances had been matched.

When matching preamps to power amps, a general rule of thumb is for the load (amp) input impedance to be at least 10 times higher than the source (preamp) output impedance to provide a suitably flat frequency response. Many prefer using a minimum ratio closer to 20 to 1, or having an amp with input impedance 20 times or more greater than the preamp output impedance.

With solid state preamps, this is generally not a problem since most have output impedance of only a few hundred ohms or less, while most SS amps have input impedance of at least 10K ohms. However, you must pay much closer attention when trying to match tubed preamps to SS amps, since many tubed preamps have an output impedance of several thousand ohms or greater. Another thing to watch is how the output impedance spec is reported, since it is sometimes limited to a measurement at a given frequency such as 1K Hz, while the actual output impedance may vary with frequency. It is not unusual for the output impedance of tubed preamps to rise significantly as the signal approaches a lower frequency of 20 Hz, because of the size of coupling capacitors used in the preamp. In these cases, a low frequency roll off can occur whereby, for the same power output, the lower frequencies drop in output compared to the rest of the frequency range, resulting in a loss of deep bass.

The good news is that most tubed amps have sufficiently high input impedance to allow the use of most preamps, tubed or SS. Also, for SS power amps, input impedances of around 50K ohms and above are common and these amps should work well with the vast majority of tubed and SS preamps.

Only a couple of manufacturers make SS amps with input impedances of 10K ohms (e.g., McCormack DNA500), and a couple (such as Pass) make SS amps with input impedance of 20K ohms. These lower impedance amps would require careful matching with tubed preamps. If you are trying to match a preamp with one of these lower input impedance amps, you should try to find information on your preamp's output impedance throughout the entire frequency range. Some manufacturer's report this information and some do not. A good source is a Stereophile review, since JA commonly provides the information as part of his measurements. Others will likely have good suggestions I have missed, but this information should give you a good starting point.
Others will likely have good suggestions I have missed ...
Not me! That was a REALLY excellent writeup, Mitch2.

I'll just add a little emphasis to your mention in the first paragraph that power transfer is completely unimportant between preamp and power amp, as well as for any other line-level interfaces. What matters is transferring the signal VOLTAGE accurately.

And although it's not directly relevant, it may be worth mentioning that high preamp output impedance in combination with high cable capacitance can produce a slight upper treble rolloff. That will usually only be a significant effect if the run is very long, and the cable type has high capacitance per unit length, and the preamp output impedance is high.

Best regards,
-- Al
So, my power amp specs indicate an input impedance of 100 kOhms, and an input sensitivity of 2.71 Vrms.  The amp has only XLR inputs on it.

Would I be correct to assume that most SS pre-amps would work well?  The pre-amp I'm researching indicates an XLR output impedance of <160Ω.  Thanks!
That input sensitivity of your amp is on the high side. Your preamp will have to put out 2.71 volts or more to run that amp at full power. BTW, what amp are you using with such a high input sensitivity?
The pre-amp specs I'm researching indicates:  XLR Maximum output level>22VRMS.
The power amp is a Krell FPB 400cx. 
Would I be correct to assume that most SS pre-amps would work well?

Nearly all preamps, whether tube-based or solid state, will have no problem driving 100K. And that would certainly apply to a solid state preamp having an output impedance as low as 160 ohms.

Regarding the sensitivity, the 2.71 volt figure most likely means 1.35 volts per leg (i.e., for each signal in the balanced signal pair), which is about average.

-- Al
My Cary SLP98P is 800 ohms impedance output, the Pass Aleph 2 has 10 kohms input impedance, will this work. It’s seems so from what I have read?,  But there are more knowledgeable folks on this forum who can advise? 
I was considering a 4m run of Nordost Red Dawn RCA unbalanced interconnects, does this effect the equation? 
How to match preamp and power amp?
To determin "if" you need passive preamp (no gain) or an active preamp (with gain). There was a test that was a very good indicator, and that was the "Revels’ Bolero Test". This cd starts off extremely quite for quite a while and builds slowly.

You do a direct connection between source and poweramp (no preamp) making sure if it has it the source’s volume is up full.
Switch everything on and press play to start "Revel’s Bolero" just for safety have your finger ready on the source’s stop button if it gets too loud as it get’s into it.

If it get too loud or is way over your normal listening level, then a 10kohm passive preamp will do you fine, like the $49 Schiit Sys. If your poweramp has 33kohm or higher input impedance.

If the sound is not loud enough even over 10mins into "Revels Bolero" then you need a preamp with gain, a Schiit Saga or Freya are great buys also, these can also be passive as well as active

Cheers George
@RPNZ: Cary Audio's published specifications for the the SLP98 show the output impedance at 440 Ohms, not 800.  Yes, I know this is an old thread.
Ok - So I have a Copland CTA301
    Output load 2. 8kohm
     Output voltage (at the time of 47kohm load) Rating: 2V Maximum: 50V
 And Caspian M2
   Input Impedance 38 kohms
    Input Sensitivity 240 mV rms

Would that work?

I need help so this topic is right for me I had m8s500 power amp I bought

The m8 preamp musical fidelity the sound is horrible didn't know it was going to sound that bad I thought that it was the right match what preamp should I buy for this m8s500 power amp



You have a MF pre controlling a MF PA and the system sounds horrible. Hmmm....MF claims the M8s is the perfect match for the M8s-700 monoblocks. I find it difficult to believe why it would not be equally as good with the M8s500. This is odd. Perhaps there is some spec that dictates it is a better match for the M8s-700 than the M8s-500 but I do not know what that would be. Certainly not one that would make it so incomaptible as to sound horrible. I would contact MF or the seller.