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

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

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.

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Ditto that you need an amp that's stable down to 2 ohms.

Bi-amping them is the optimal solution.

Properly amplified with with a great preamp, they punch waay above their weight class. You should also have a good sized room to really let them strut their stuff.
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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.

Cheers George

Maybe Ralph's (Atmasphere) MA3 could do the job if you want tubes.