Sorry, my mistake -- that circuit was in the Nakamichi 600, not the 500.
--When setting up a deck for a specific type of tape, the Dolby level match is the final step after bias and equalization. Setting bias is a particularly painstaking process on any two-head tape deck; EQ and Dolby level, somewhat less so, but 15-20 minutes is a reasonable amount of time to set aside for the whole procedure. The same process takes only about two to three minutes with a three-head deck since you can vary the setting and observe the results in more or less real time, without continually recording, rewinding and checking the output. Also, the optimal gap width for a record head vs. a playback head are quite different from each other. I'm still fond of analog tape (I've been transferring some of my own, decades-old recordings in the past few years) but two-head decks are always a major compromise.