Biggest issue is tube preamp running into a digital amp, something with input impedance of 10K or less.
In that case the highs are frequently rolled off because the load is too low for the tube output. The problem is made worse if you are using two sets of outputs from the tube preamp, say for home theater or to drive a separate sub woofer.
Many tube amps (VTL for example) have an input impedance of 150K which works fine for both solid state and tube preamp designs.
I just researched this topic yesterday. The output impedance of the preamp should be within 10:1 ratio to the input impedance of the amplifier. Thus, the preamp is the lower number in the equation in this example. I have a solid state amplifier which provides 50k ohm input impedance. My tube preamp provides 592 ohms output impedance. Given the formula above, the two units are compatible. Hope this helps!
Al, (Almarg) has answered this question numerous times. Maybe he can find the response(s) and copy and paste them here.
Actually I found one of Al's posts:http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1323999458&openusid&zzAlmarg&4&5#Almarg
1:10 min. ratio output to input impedance. THis is the accepted ratio for minimal impact on freq. response.
Eg: pre out= 100ohms matched well with amp input impedance at 1kohm (1,000 ohms).
My CAT pre ouput impedance is listed at 75 ohms, so will match well with any amp input impedance of 1K ohms and above.
The general rule of the thumb is, the input impedance of the amp should be ~15 times or more than the output impedance of the pre (or the source unit, generally speaking). That's generalising, of course.
As Aporter mentions, amps with 10k inputs are a tad difficult to match, while others with 150k are very easy to drive.
You can be sure of having a perfect match if you use the same brand of amp and preamp. Not only will you have an electrical match, but you will also have a sonic match. I have mixed amps and preamps on many occasions and came up with some interesting sounds. However, when using the same brands I always hear a rightness to the sound that can not be achieved by mixing brands.
Mofi, thanks for linking to my post.
As I indicated there, and as you realize but others may not, it is not as simple as taking the two published numbers and seeing if they differ by a factor of 10 or more. The published impedance numbers are typically at a mid-range frequency (e.g., 1 kHz), and can be much different at other frequencies, especially in the case of most tube preamps.
Here is a quote from another thread
on this topic:
Ideally the input impedance of the amp should be 10 or more times greater than the output impedance of the preamp, at the frequency for which preamp output impedance is highest. Otherwise audible frequency response irregularities MIGHT result.
If, as is often the case, the highest output impedance of the preamp across the audible frequency range is not known, and only a nominal output impedance is specified (perhaps based on a frequency of 1 kHz), I suggest using a ratio of 50 or more, and preferably 75. Many tube preamps, and some solid state preamps, use a coupling capacitor at their outputs, which can cause their output impedance to be much higher at deep bass frequencies than at higher frequencies.
Impedance incompatibilities are most likely to be encountered when using a tube preamp with a solid state power amp.
If the power amp has an input impedance of around 47K or more, it is unlikely that there will be an issue, even with a tube preamp.
If the component has been reviewed by Stereophile, the measurements John Atkinson usually provides will indicate its worst case impedance across the audible frequency range, to which the 10x factor can be applied.