What's a Triode configuration?

Don't limit yourself to Audio Research. There are many tubed brands (Sonic Frontiers, BAT, deHavilland, Joule, Supratek, Conrad Johnson, Rogue, etc.) which will offer you better performance at comparable pricing. Check out the number of used ARC pieces for sale. Serious audiophiles don't use them very long after hearing the sonic deficiencies in their own systems.
Good luck.

Hmm...in 25 years in this insane obsession...er...crazy hobby, I've seen only one tubed preamp, NOT an ARC, with a pentode in it. Said another way, ALL active tubes in ALL tubed preamps (but this one) were all triodes. I wonder why your suggestist even mentioned this.

Just to be confident, I suppose you could ask if the preamp you're considering is all triode.

ARC makes great stuff. Conrad-johnson and others, too, do.

Understand that some tubed preamps have much-higher-than-normal output impedance. This will somewhat limit your choices in length and types of IC and poweramps driven. Ask about that, too.
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Your last paragraph is probably the most important factor. I'm going thru it right now. I have a very nice preamp that unfortunely is a mismatch with my amp. Even after lowering the amps input impedance the pre has too much output impedance and overloads the amp causing a distortion.

Kora makes a nice tubed pre-amp, called the Equinoxe, I believe. I had very good success with its predecessor, the Triode. I ran the Triode with both the (SS) Plinius SA-100 and the (SE) Melos MAT-185, both with good success. There is a Triode for sale on the Audiogon right now for $400, I think.

Of course as Jeffreybehr points out all low level signal amplification is done with triodes, often the ubiqutous 12AX7, and its clones. The alternative to a triode is the pentode, which is used in power amps that have transformers. The extra grid in the tube is used to apply feedback from the transformer secondary winding so as to correct frequency response deficiencies of the transformer, and, as a secondary purpose, to reduce distortion.

IMHO your shopping list should not be constrained by arbitrary factors like the type of tube. You might not like the sound of triodes.