As a general rule, you want to make sure that the input impedance of the amp is at least ten times the output impedance of the preamp.
Almost all tube preamps (except those with output transformers, of which there are few) have high output impedances in the bass, e.g., 4,000 Ohms, which causes bass rolloff (decreased bass) with some amps. High-end solid-state amps often have input impedances of 100,000 Ohms, so it's not always a problem, but there can be a problem with certain solid-state amps (e.g., classic Rowland, many studio amps).
Almost all tube preamps (except those with output transformers, of which there are few) have high output impedances in the bass, e.g., 4,000 Ohms, which causes bass rolloff (decreased bass) with some amps. High-end solid-state amps often have input impedances of 100,000 Ohms, so it's not always a problem, but there can be a problem with certain solid-state amps (e.g., classic Rowland, many studio amps).
