I have Infinity Kappa 9 speakers and have driven them in a biamp configuration with two Sunfire Signature stereo amps. The Sunfire Signature amps are much more powerful per channel at 4 ohms than the TGA 7400. The Kappa 9 will dip below 1 ohm and will make some amps call for their mama. They need lots of power. The Sunfire Signatures did a very good job, but I tried four Carver Silver 9t monoblocs and they were clearly better on the Kappas than the Sunfire Signatures. With that said, you haven't even come close to the Kappa 9 capabilities with your current sunfire. The Sunfire Signatures are rated at 1200 wpc at 4 ohms and the Carver Silver 9t are rated at 900 wpc at 4 ohms. The Carver 9t were clearly superior, but, you need four of them to make the Kappas sing.
I would suggest you try the Krell. It will hold its value and you won't lose any money if you decide to sell it. The Sunfire will depreciate precipitously....get rid of it now while it is still fairly new and you can still get a good price for it. It is a decent amp, but it had nowhere near the testicular fortitude needed to properly drive the Kappa 9's. Once you get some serious amplification for the Kappa 9's, you'll find out that you've just gotten a helluva set of new speakers. I own four pair of Kappa 9.