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

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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.

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?

Krell MDA-500

http://krell.hifihaven.com/model/MDA-500

http://krellonline.com/assets/support/MDA300500_MAN.pdf