Krell KSA150 vs KSA100S & KSA100 for Inf Kappa 9

I've been struggling for a year now to power a pair of Infinity Kappa 9's or should I say pwering the woofers. I've bought 2 Parasound HCA-3500's and Adcoms Flagship model. Neither can run the Kappa 9 woofers at high volume levels for a length of time. Some of you said right in the beginning to buy a Krell. Well now I'm ready to make that purchase. I'm only going to use the amp to power the 4 woofers. What is the comparison between the KSA150 and the KSA100S and the older KSA100? I can spend up to $2,000. Also should I hold out and spend a little more on a KSA250 or KSA200S? Any advise will be greatly appreciated.