Most transparent, detailed Audio Research Preamp

If I'm looking for a used Audio Research preamp (not the Reference series either!) that is detailed, open and transparent, which model should I look at?
Any suggestions are appreciated.
Well, depending on taste and budget, SP9 MK III, LS3, LS16 Mk I or II, LS25 Mk 1 or 2. All are (to one degree and another) all of the things you're looking for-- although they all sound different from one another, and the LS16 and LS25 models are quite a bit superior to the older models.
There has been so much discussion on ARC preamps, the SP series, the early hybrid LS series, a return to an all tube line stage, the LS5, and then back to hybrid designs again. Do a search in the archives here for a lot of valuable information. One thing about ARC preamps, there has not at all been a consistent house sound and later models have often been very disappointing compared to those of 5-10 years before.

Terms like open, transparent, detailed are used so heavily that it's pretty tough to define a reference point for each of these characterizations. The one strong attribute with ARC designs has been dynamic contrasts. And their presentation is more of an "out in the room" rather than "back at the speakers".

Other than the LS25 models, all the models mentioned by Swklein I would classify as part of the ARC sterile series. There might be a little more detail, but often this comes with fatiguing forwardness in the upper midrange, lack of tonal coherency and a significant loss in 3-dimensionality and decays/bloom of the notes. When you hear a few of the ARC models float piano notes in space, these other models are impossible to listen to.

As an owner of the SP-10 for 8 years and then the LS5 for 7 years, I put much effort to audition all the models in between and it came down to avoiding the hybrid products like the SP-11, SP-15, LS1, LS2. Sure, they brought on some refinements in frequency extreme extension and less background noise, but at a cost of musicality and realism. The LS3 is fully solid state; you can forget about getting any of the magical ARC sound here that is often associated with older ARC designs.

The LS5 does a wonderful rich sound that the LS15, LS22, and later LS16 don't even begin to match. You don't mention the rest of your system, but rather than lock yourself to only ARC products, I think that BAT and Counterpoint would be worthy of investigation here.

I have had a LS7 which I found to be a little dull sounding, moved that out for a McIntosh C37 nice improvement in detail and soundstage. In my main system I just traded out a LS25MK1 for a First sound deluxe MK11 what a change!!! everthing opened up, detail, soundstage etc.The downside(if you want to call it that) is that I also started to hear my power amp's shortcomings, a Bryston 4BST which led to a new power amp. So you may want to consider a look at a First Sound pre.