Bias is HF signal above audio band (usually 50-100kHz) used to reduce distortions of the tape. Too much bias on the other hand causes drop in high frequency response. High frequency content in music also plays role of a bias and by controlling total sum at given moment much better extension can be achieved without increasing distortions. Inexpensive tapes can sound like best chrome tapes. I had this long time ago in my Aiwa Deck and it was called HX-PRO. It is "servo" on the bias. Tapes recorded on this deck sounded much better than the same tapes pre-recorded.
