Mark VI can be very sensitive for how they are setted up, I mean for key action, spring tension, overall keywork setting, pad, resonators etc etc.
(Obviously the pad sealing must be "perfect")
It's only worth to verify the overall setting of the horn.
Some settings can have an indirect effect on some other aspect of the horn.
When you are able to test many horns (in the same session), another good test is to switch the necks between the different instruments. It happens in most of cases... the "best horn" has the "best neck" and if you put the "best neck" on another horn, you'll probably feel a kind of improvement.
You can a great variabily on the necks also if the neck are original, never demaged, never repaired... because there was a great variability also during the neck manufacturing (as well as for the whole saxophone manufactoring).
@Ben: have you consider to try some alternative necks for you 98xxx (also a custom neck)?
Are you looking for just a "darker" horn and/or for a sort of "more resistant" horn and/or also smoother/slower in response?
Just for talking about...