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.
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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.
The dips are sharp and right at about 30hz and 7000hz. Contrary to popular belief, the published sensitivity is actually pretty accurate. It's probably the impedance dips responsible for the notion they are 89-90db. A properly matched amp will drive them to very loud levels very quickly. It is well worth accommodating them.
Al (Almarg) is right Fartman. You need and amp that will deliver big current, which means one that can almost double it's wattage from 8ohm to 4ohm to 2ohm. This means BJT (bi-polar) output stage solid state type amps, with big power supply, eg: old big Krells.
Because the Kappa 9's go down to .7ohm in the low bass and .8ohm in the upper midrange. And if there are any bad negative phase shifts in these areas those impedance figures will be even lower.
This means Tubes and Mosfets need not apply to drive these successfully.
George, I recently sold three Acoustat TNT200 and a 120, all fully rebuilt by Roy Esposito in Florida. These amps are pure Fet, designed to drive the Acoustat line. Even in mono mode, they had no trouble driving the Kappas. The reason I sold them is because I came to my senses and realized my LSR&D 101s and 102s are superior in sq. I got them for an active JBL system. Anyway, it seems that the very nature of mosfets would make them especially suitable for low impedance loads, no?
Simply put NO, they will drive them but will run out of steam (current) at those loads below 2ohm, especially at the below 1ohm points discussed, BJT's (bi-polars) are for these speakers.
And I believe most Infinity's were voiced around Krells, as they were one of the only amps around at that time that could drive these sort of loads "with ease".
But as I said, the Acoustat TNT200 and 120 were designed to drive low impedance loads. In stereo, the 200 doesn't even break a sweat. In mono, they get a little warm. That's driving the Kappas. They have the Hitachi T03 2SK135/J50 and the 120 has the 2SK134/J49. Roy, who was a principal engineer at Acoustat and instrumental in designing these amps assured me that they can easily handle the Kappa 9s. He was correct.
My assertion regards the inherent nature of mosfets to thermally self-protect by reducing conduction as they heat up. Seems ideal in low impedance circumstances.
The Acoustat TNT200 has 7amp Mosfet output devices at 6 per channel. A max of 21amp peak, Krells can have 15 - 20amp output devices at up to 4 x the amount, do the math.
The TNT will drive the them ok?, but where the Infinity's load down low and in the upper mids is you need a amp with a Krell like ability to get the best out of them.
Now that would up a rocket up them 24 x 20amp BJT per channel, overkill yes.