What is HX-Pro?
Dolby HX Pro is not noise reduction and does not use compression or expansion. HX Pro is a technique developed by Dolby Labs to increase tape headroom by decreasing the bias when recording signals with a large high frequency component. This allows better transient response, particularly on less expensive tapes, and requires no processing when the tape is played back. Dolby HX tapes can be played back on any system with no decrease in quality. (courtesy of AudioFAQ)
How does bias fine tune work?
The bias fine tune only changes the sound during the recording process. Variable bias is a compromise to adapt the deck to different tape brands. It is most useful for type 1 (normal) tapes. You make a recording, and then compare in playback to the source. If the recording sounds bright, turn the bias knob clockwise 30 degrees and make another recording. Dull? Turn the knob counterclockwise. You can also test by recording FM inter-station hiss, but keep Dolby off and keep record levels at about -20dB. (courtesy of Tom Brucker)