Used Audio Research Preamp Options

I had previously owned an Audio Research SP-3 and LS-3 and earlier this year decided to upgrade preamps. After considering some of the excellent suggested used ARC preamps mentioned in the above responses, I decided to pick a new ARC LS-17SE on closeout from an authorized ARC dealer for $2795.00. Overall, I felt this was the best deal for me as the preamp listed for $5995 and was new in a sealed box from a well reviewed and authorized dealer. This unit was discontinued in 2015 as part of ARC's ever changing product lineup.
I thought although there are some good buys available for some used ARC preamps, issues such as lack of warranty, possibility that tubes would need replacement and that given a need to replace caps after (in some cases) within 15-20 yrs., I felt a new unit was the way to go.
Since these units are still available at this price, you may wish to consider that option as well. I have been very satisfied with this unit's build quality and sonic performance and would not hesitate to recommend it as a best buy in the price range that you are looking for.

You may want to do some further reading on the pros and cons of a preamp vs. direct connection. One approach is a passive line stage, i.e. a device which provides basic input switching, volume controls and outputs but no amplification of the signal.

See the below link for a further and lengthy discussion on this and other approaches to removing a preamp from the chain.

http://www.high-endaudio.com/RC-Linestages.html

Another approach I have tried with the Mytek Brooklyn DAC is to use it for a preamp, removing the ARC LS-17SE from the chain. This DAC has the necessary setup (volume controls, etc.) to allow for this approach. Although it sounded pretty good, I reverted back to using the ARC LS-17SE as  it had more inputs and a more natural soundstage and warmth. I suppose I could live with the Brooklyn DAC as a preamp but since I have the ARC unit I will periodically experiment with it vs. the DAC as a preamp.
I am not aware of any ARC preamps which allow a complete bypass of their circuitry, only a home theater processor button such as on the LS-17SE which allows for a bypass of it's volume control to allow a separate home theater processor to control the volume.
Although in theory removing things from the audio chain should improve sound, in practice this may not always be the case.

I checked the ARC LS3 owner's manual and found that the Direct/Normal switch acts as described above and bypasses the Balance, Mode and Input Selector switches.
As I recall from when I owned an LS3, there was no great sonic difference using the Direct Mode vs, Normal. This is probably because you are still subjecting the signal to some of the preamp's electronics and therefore this is not a truly passive line stage circuit.
In any case, if you want a preamp vs. a passive line stage, this should not be a deciding factor as to which preamp to purchase.