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Since I got my King 1240 I've noticed that slurring on the open partials all the way to the top of the staff has become far easier for me than before on either my 186 or Kalison DS. Unfortunately I can't chalk it up to more practice time. Is this the old rotary vs. piston debate regarding response? Or does it have something to do with the lead pipe going directly into the valve set (since it goes in the "back way" on the DS)? I admit my slurring abilities in the staff were never very admirable, but it seems downright easy now, any thoughts?

The whole mouthpiece/gap thing affects this too. My Marzan had no receiver, just the mouthpiece straight into the leadpipe. That was a VERY easy tuba to slur with, and had great response.

I will just add on to the good responses so far....
In my experience, the leadpipe/mouthpipe has a considerable effect on response.
You can easily notice this effect when you swap back and forth between "old style" and "new style" goosenecks on King sousaphones. Just look at the stories here from tuba players who have swapped out an old leadpipe for a new/different one and how much it changes the response (and sometimes even intonation) of the tuba