I would need a pro grade outboard Dolby S unit to experiment with it. I don't think I would want to go in this direction.@inna
I don't blame you and I wouldn't either! Plus IIRC most outboard units are actually Dolby SR, which are meant for reel to reel. I know Pioneer made some great (and expensive: +$1000 when new) Dolby S machines but beyond that I didn't keep track of them.
ZX-9 should be better than 682ZX in terms of dynamics and resolution but 682ZX is a forever classic Nak.One thing to look out for with any of the Nak 3-head machines is to make sure they record in *both* channels prior to purchase! The Nak record heads tend to fail leaving one channel out in record mode. They are rare and expensive- I've seen them for over $700 on ebay.