Compressors are dynamic processors. You can control the the volume of the audio signal but you can do more with a compressor. You can make it pump the signal for a rhythmic feel.
It can also make the audio signal 'flat' and take out the life of it.
It's up to you what you do with a compressor. You have all kind of compressors. VCA, Optical, Tube, digital and hybrid ones with all of there own character and best purpose for an instrument.
Preamps are gain processors. They are essential to raise the volume of i.e a microphone or an instrument to have the best signal to noise ratio.
Preamps are on every channel of a mixing board but ofcourse you also have seperate preamps. They come like compressors in all sort of flavors. Transistor preamps, tube preamps and hybrid versions.
Preamps are essential for the sound of the instrument because they color the signal always. There are neutral preamps and they are very welcome but most of the time people look for 'that' preamp to color the sound.
See which compressors & preamps I have for sale on the STORE page !