pre/power matching?

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.

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

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.