I, too, am curious about what Phil has to say. But to me, the Def 4s are a major step up. Even though they look and are sized like the Def 2s (a look that I like and a size that I can live with), everything is new. Here is an embelished version of what I wrote at the end of October after getting my Def 4s 10 days previously. I have not changed my opinion. . .
The engineering is amazing. (1) The integrated amplifier can be unscrewed and popped out for service if necessary. (2) Similarly for the bass driver. (3) The aluminum base that supports the downward-firing bass driver is a work of art. (4) The Neutrik Speakon connector preserves the geometry of the cabling and accommodates a selection of connectors for each different type of speaker termination. (5) Bass can be tuned to the individual room and is better integrated and more tight, articulate, and palpable with the down-firing sub vs the rear-firing, split-tuned array. Etc. Everything has been thoroughly thought out.
The highs and lows are more extended. Deep organ notes are palpable (when appropriate). Resolution is better, but the sound is also smoother, more organic, and even a little more dynamic. The soundstage is deep and wide even though they are within 4" of the back wall. Off-axis imaging is great; maybe it is the way that Sean set them up on my 24' long wall, but the image does not change very much whether I am between the speakers, sitting in front of one of them, or standing against one of the side walls. And yet, everything that I liked about the Mk2s is still there. The sound easily fills my living room and the extended 45' long main living space of my house without a change in tone or sound quality when I am in a different room. I have listened for hours without any fatigue.
