Electric violins (and any electric instrument for that matter) cannot be decently amplified and processed without using a preamp.
You need a preamp to get your signal clean. This is even important if you are only starting to experiment with electric violin sounds, and you just rely on a simple pick-up on your regular acoustic violin.
A piezo without using a suitable preamp generally results in an unclear, rough sound. This occurs for example when a piezo pickup is plugged directly into the electric guitar equipment.
The solution is to choose a preamp that matches your violin (acoustic or electric) and your piezo. Most suitable preamps are especially built for acoustic instruments. The preamp will control the sound that is sent to the amplifier and amplify the level.
A preamp takes the pickup signal and translates it to match the requirements of the sound system, whether it is an amp, a PA or an effect device.
These systems usually use low impedance sources while most pickups have very high resistances. Preamps are used to boost the signal level and lower the impedance.
A DI box allows a player to send the signal using balanced microphone cables that are easier to use and connect to live sound equipment.
Balanced cables have high RF interference resistance, even at long distances, resulting in a cleaner sound.
Preamps offer sometimes additional, advanced features, However, you don’t really need these features if your amp already includes them.
Some preamps allow a player to mix several signals and apply effects on them. This is for example useful when you have both a piezo pickup and an internal microphone in your instrument, and you want to have full control over the balance between both.
Many preamps have an equalizer built in which makes the sound better before processing. An effects send is useful if you want to have control over your effect balance on the preamp level, it is not essential if you have these features on your guitar cabinet or if a sound man can take care of that.