Evaluating components at a show can be a bit tricky. If the sound is really good, you can be sure that the speakers are at least capable of delivering good sound. But, if they sound bad, it is hard to know if the set up is the cause. If the same speakers sound bad at different shows and in multiple rooms, that does give you a hint. I have heard the Def 4s at several shows and in many rooms. Often, it does sound quite aggressive and unpleasant. But, in a few setups, it sounded quite good, which, to me, means it is capable of sounding good.
For some reason, most of the setups I heard used powerful solid state amps. I would think, given the efficiency of the speaker, it would be something that could be matched to lower-powered tube gear (my preference).
Johnk nominates the 80+ year old Shearer system as the best ever made. It certainly is a contender, as is a number of Western Electric systems, and more modern clones of such classic systems. I have been listening a lot to custom systems being built these days by Deja Vu Audio utilizing an assortment of drivers from Western Electric, YL, UPC, etc. I like what can be done with such drivers. The current build that is being fine tuned utilizes 18" Jensen field-coil woofers (reconed with original, unused cones and pleated surround), a WE 713a compression driver and WE 32 horn, and RP302 tweeter, and a vintage WE crossover. I wish everyone could get a chance to hear this sort of system. It would certainly surprise those who hate horn systems based on their exposure to Klipsch or common Altec systems.