Each cassette player (and tape) has its own ideal biasing. My Nakamichi 700 had various adjustments to optimize to each tape as you recorded. Some of this was done to optimize the Dolby B encoding (which when done "right" like on this machine was really quite good). It was also needed to account for the narrow tape and slow tape speeds. (IMO Dolby's downfall was over selling for use on cheap recorders/players with the effect of it sounding more like a treble filter than anything else.)
Trying to play back on another machine, even the hand assembled version (Nakamichi 1000) didn't sound nearly as good. Friends always wanted me to record for use on their machines and it always sounded poor, especially on those cheap machines.
Bottom line, high quality cassette recordings are forever tied to the machines they were recorded on. This was the dirty little secret of cassette. I should have bought a Revox and just ate the extra cost of the tape. (I worked at the college radio station and had access to their entire library.)
