The general rule of thumb is the input impedance of the amp should be at least 10x greater than the output impedance of the preamplifier - this is almost never an issue when using a tube amp which genrally, but not always, 100kohm input impedance. Generally only a consideration with 10Kohm amps (some Pass amps for example), but it is said that the greater the differential the better. Ofcourse, theory ain't worth a darn compared to trying it out and "specs" can be misleading and don't always tell the full story.
