Late to the party here, but I have a few thoughts (apologies in advance). Like the OP, I also recently switched back to cone speakers from planars for the same reason. While the planars did some things extraordinarily well, there was a trade-off that had become more obvious to me over time, and I interpreted it as having something to do with "slam". The problem I see is that, until this thread (thanks SimplySound), many of us had heard these terms and defined them for ourselves.
For me, "ring" is a negative - if you can hear the speaker rather than the music, that's ring. I have seen some speaker designs which tried to use the cabinet as a resonator and I never bought in to that idea. A speaker that introduces its own resonances is not an accurate speaker. This is one thing I like about planars - no box to resonate. Similarly, a speaker in a box must be solidly built and well damped (physically), in order to avoid "ring". Of course, the drivers themselves can also ring due to poor driver design and/or poor amplifier control.
The way I've self-defined dynamics comes from my background as a musician - essentially dynamics is the difference between the loud parts of a piece and the soft parts - without respect to changes in time. In other words, is it soft or loud at any given moment?
Slam would simply be a measure (or perhaps perception) of how well a system can express dynamics over a very short time frame. In other words, how quickly it responds to dynamic changes at the fundamental frequency level. Good performance in this respect is fairly easy to achieve at high frequencies, but is increasingly more difficult as the frequency goes down. That seems counter-intuitive - lower frequencies (fewer cycles/second) should be easier to keep up with, no? No. As frequencies go down, power demands and volume of air go up exponentially, so the physical systems needed to reproduce those frequencies must be much more robust, which makes them tend toward being slower.
So in my interpretation, where fast response at high frequencies might be recognized as "sparkle", at lower frequencies the same phenomenon becomes "Slam". It also can't be overstated that distortion will quickly degrade any sense of slam, "smearing" the impact, so good slam must be not only quick, but clear.
So size does matter! 
