Whats the range of specifications on wow & flutter and harmonic distortion of bbx's that are considered to be..... good - average - poor ?

I would say average for most "quality" boxes Drew.
Some better than others of course.
I think there were quite a few with around a 0.09% rating.
I know the Aiwa CS 770 and CS880 for example had a very good 0.038% wow and flutter rating.

most boomer decks are pretty basic - these machines didn't have hi-fi at the top of the list.
There are some notable exceptions: the decks on the JVC RC-M90 & the Sony ZX-7 spring to mind...