Bill
What everybody is telling you about the speakers being defective in some way is true. It's the Kappas, not the amps. The Denon had more than enough power, and if the Kappas were right the Parasound would launch them. Something is wrong with the Kappas, and it sounds like they are shorting somehere. The guy in Tennessee is Bill Watkins, who worked with Arnie Nudel at Infinity and designed the Watkins dual voice coil woofer. He still repairs stuff. I bought some RS 2.5s a couple of years ago which Bill Watkins had just refoamed. You can find him online.
Klaus Polig's Infinity-Classics.de website has a lot of Infinity info. Here's the address for the Kappa 9 page which has a schematic.
http://www.infinity-classics.de/infinity/models/Kappa-series-1987/index-Kappa.htm
If you haven't already, post your question on the AudioKarma website. They have an Infinity only forum.
It's the speakers, not the amps.
Plus if you keep them you are going to have to bi-amp them. If you are putting all that juice into them the mids are a little delicate, and they won't handle the load after a while. I just asked my friend Jay who works on them about your problem and he says you will have to modify the crossover. It forces the woofers down to 1 ohm. So if you want to avoid all that, sell the Kappas and buy some Vandersteen 3As. They may not play quite as loud, but they will hammer the Kappas in every other aspect.
Good luck, Martin.