Preamp under $2,000 used

Definitely the Ayre K5xeMP, basically the latest iteration, though it may be a bit above 2K used.

To add to that, I own the Ayre k5xeMP, and I love it. It has an open, smooth sound, maybe a touch of sweetness, has one of the best volume controls I have ever had the pleasure of using. Fully balanced, dead quiet. Replaced a Dehavilland Ultraverve 3 and is better than it. Though I may soon put it on the market as I am very happy with my new tube integrated and I am not planning to keep my SS separates.

Well after much deliberation and looking for good deal on used preamp I found an Ayre K-5x it it sounds fantastic! Whole new level of refined sound, much sweeter highs and more extended  Vocals are very smooth and natural. I notice pianos, horns and live music sounds much more realistic.Live guitars sound like you are there in an intimate live setting, just amazing when listening to Dire Straits live. 
Now waiting to trade RCA interconnects for XLR for the next step up and better gain, Ayre recommends XLRs for best sound. I'll report back. 

 PS Audio 5.5 
It`s over 30 years old, but will hold It`s own against anything under $2,000 used, And would sound even better with a rebuild.
The phono stage is first class.
Look at all the reviews on it.
They are very rare to find.  PS Audio doesn`t fix them anymore because they don`t have the parts.   I have one that is mint, and will never sell it.

I always think that the individuals ears and what they hear as the owner of the stereo system is the most important. You have to consider your room size and your favorite gender and the type of music you mostly listen to and if it sounds good to you sit tight because you’ll never cover all the bases.