Dolby levels may be standard but tapeheads sensitivity and output voltage from the first playback amplifier stage apart from being different from deck to deck, ends sometimes in different filters or filter configuration .Sometimes there's a second stage then the bias trap then the tape type corrections...it does differ from deck to deck and sometimes you get different results.There's also a third op-amp amplifying stage in the dolby chip to help equalize the dolby levers.Yamaha kx 990 and nad 6300 used that playtrim in their advantage not only to equalize tapeheads wear but also to deliver equal dolby levels with unequal playback levels on each channel .The best match i found between different decks for dolby c encode decode algorithm was pioneer ct 777 and technics rs az7 although they used different dolby chips from Toshiba and sony...and not all dolby chips were born equal...