The term '6 RMS ___ VU' is a reference to the popular audio measurement system known as Root Mean Square (RMS). RMS is used to measure the average power of an audio signal, and is often used to compare the loudness of different audio signals. The answer to the crossword puzzle is 'RIV', which stands for RMS Voltage Unit. This unit is used to measure the voltage of an audio signal, and is often used in conjunction with RMS to measure the loudness of an audio signal.