Ohlala gives good advice with his 100X formula, although, sometimes satisfactory results can be achieved with less difference than that. Generally, tube preamps have higher output impedance than solid-state units, and transistor amplifiers often have lower input impedance than tube models and that's where the potential for mismatch creeps in. If you own a unit now that you intend to keep and use, then the 100x formula should be a consideration in your search for a matching piece. I usually like to see around 50k input impedance in a solid-state amp, as that will mate well with many tube preamps. Then, using that 100x formula, you can go up to 500 ohms output impedance (or thereabouts) in the preamp without a problem.
