We used the MM(basically a video machine with audio-only heads and electronics) and MR-70, only occasionally 300 and 350. The workhorse was the MR-70. This was a pleasure to operate. The variable speed wind/rewind was unique, and made all the difference in the world. I didn't have to risk slicing a finger slowing tape the old-fashioned way ! I think the electronics had more head-room (dynamic range) than the 350 and AG models.