How to match ohms with amps & preamps

How can you avoid pre-amp/amp mismatches? I am speaking of ohm matching, I know what ohms are but I do not quite understand their role in amp/pre-amp matching. What parameters are ideal? In addition, what happens when you mix a warm amp with a dry preamp or vice vera? Thanks.
An excellent question. We can all save ourselves from some unnecessary experimentation, expense and aggrevation if we just do the math. A simple solution is to compare the same manufactures preamp/amp specs and try not to deviate to much when mixing and matching. Just slapping any tube preamp to
any solid state amp is not the pancea many claim it to be. It can work wonderfully, but is much more diffucult than it appears.

According to David A. Wilson in an article for the Absolute Sound (volume 14 issue 63 january/february 1990) "For example, it is obvious that the most trouble-free combination, with regards to Xc factors, would be a low-output impedance, direct-coupled preamp to a low-input impedance, direct-coupled power amp."

Guidlines For the Consumer

"HIGH"* ....... OVER 600 OHMS...........100 K OHMS..........
"MEDIUM"*...... 200-600 OHYMS.......20 K OHMS TO 100 K OHMS.
"LOW"*........ LESS THAN 200 OHMS...... LESS THAN 20 K OHMS.

*IN DAW's opinion, no official standards exist.

It is my understanding that due to a fire none of the older copies of The Absolute Sound are available. As such I hope all concerned parties will forgive me for this excerpt.
There are other opinions on this topic. I believe that Jeff Rowland believes in a more uniform power transfer and the Spectral gear has some unique requirements. Others have suggested a 1 to 10 ratio. IMHO the best suggestion would be to seek advise from the manufacturers of all your equipment. Of course this should be the job of your local dealer, but I would not take this advise with out confirmation.
Thanks. At least that gives me something to go on. Does the same apply for digital sources plugged in directly to an amplifier?
It depends on how the digital source outputs to the amp, if it has a traditional analog volume control with op-amps or some other kind of buffering it should be the same if not better considering the fact you will have one less interconnect in the system (unless you end up using longer a interconnect). If you were using a passive pre-amp things get more complicated, as the impedance will change with the volume control setting . Cables will always be a consideration when connecting components, more so when going passive. The new all digital (Tact, etc.) systems should present less of these problems.
Historically, the accepted ratio was that the input impedance of the device receiving the signal should be a minimum of ten times the output impedance of the device sending the signal and the greater the difference the better.But, as with every rule there are exceptions, certain Krell and Rowland models are optimized for currnt transfer and signal transmission is optimized when the two impedances are matched.