Preamp tubes take the small signal from your pickup and amplify it to "line level." As the signal passes through, preamp tubes influence the various EQ stages found in your amp, such as distortion, gain boost, tremolo and reverb. In most amp circuits, it is common for each stage to be controlled by one preamp tube. Therefore, preamp tubes will impact your tone more than any other tube change in your amp, especially in the V1 slot (closest to your input jack).
Additional options for your preamp tubes when needed:
Balanced: When the plate current draw of each triode is the same, we refer to that as having balanced triodes.
High Gain: Tubes that measured to have higher gain.
Matched: Each triode of the preamp tube has matching gain.