need a power amp to run a set of infinity kappa 9s

I did many years ago. Current is key. Find something that will double down in to 4 ohms, and even 2 if you can. Think balls. That will get the best from your Kappas. But careful with those tweeters, they can run hot. Something soft on top or dark - Classe comes to mind - might serve you best.

Any MOSFET based amplifier will drive those kappas no problem. ADCOM, HK citation, Classe, Krell, McIntosh 7200. Among others. Regards.

I have the Kappa 9s. You don't need huge power. What you need is the ability to handle low impedance. I can play mine 'LOUD' with my Son of Ampzilla 80wpch amp that delivers 250 watts into 2ohm. Even my Grandson of Ampzilla at 40wpch has no problem driving them, being similarly capable. But if you want concert hall levels, I use my LSR&D Superamp monos at 300wpch with 900 watt peak ability. The amp really has to be stable to 2ohm or lower to sustain relatively high spl without being over driven.

As I recall, the Kappa 9's have an "extended/normal" switch, which controls their bass extension. In the "extended" position (which IIRC was the most commonly preferred setting), their impedance goes down below 0.8 ohms at multiple bass frequencies, where lots of energy is typically required. Correspondingly, they were often referred to as amp killers, as you may be aware.

The "normal" mode is not quite as severe, but still very challenging as I recall.

Also, while I don't know what their phase angle behavior is in the bass region, it wouldn't surprise me if it was significantly capacitive, adding to the severity of the load.

As some of the others have said, you will need an amp or amps that can handle VERY low impedances, and supply large amounts of current.

Regards,
-- Al