Output sound is low or too high.

Adjust the output level on your DJ application.
THRU output cannot be adjusted.

When using rekordbox
Adjust it on rekordbox preferences.
Open [Preferences] → [Controller] → [Mixer] and change [Output Level] setting.
Clipping is likely to occur if the level is too high when applying FX or mixing. Clipping will enable the limiter to prevent distortion, but at the same time it will spoil the attack of a sound.