My turntable comes with the plastic cover. Great concept for keeping dust particles away from the spinning record.
However, some claim that this cover serves as a microphone, amplifying certain frequencies produced by the speakers (typically, lower frequencies). The frequencies thus amplified by the plastic cover affect the tonearm, cartridge body and the stylus, and interfere with the accurate tracking of the record grooves.
So my question is: keep the plastic cover on while playing records, or get rid of it and accept the fact that the dust will collect in the grooves during playback?