If you like the King mello just wait until you try one of the Yamahas.
I have an old King 1120 as well as a new Yamaha 204M I have also played on a Yamaha 203M. The biggest difference I found between them all is the back pressure involved. The King feels the most restricting followed by the 203M and that leaves the 204M which allows you to fill that instrument with a larger amount of air with very little distortion at a double forte volume or more if you have it. The 204M is a dream to play. The distance between major thirds on th 204M seem to be closer to one another which seems to make them easier to attain also. With this being said it may back up your theory on new vs. old.
In case you were wondering about my mello experience I have played mellos for 3 years with The Cavaliers, one year with the Cavalier Alumni Corps and for 2 years now with the Classic Cavaliers. I will never deviate from Yamaha brass.