Preamp under $2,000 used

Audible Illusions M3A.

There's a Pass Labs x2.5 that I'd think would be a significant improvement in your system and is under $1500.  I'd put this in the "you just can't go wrong with this" category given your price range and goals. 

However, since you're also open to ideas on the tube front, for $2k (or maybe less with a metal case) you can have Don Sachs build you one of his SP14 preamps.  If you call him and discuss your setup and needs he'll probably give you some good thoughts, and as a big bonus I believe you have a short trial period so little risk involved.  This is what I'd do as this could potentially get you to the "truly special" product category, so I'd encourage you to at least give Don a call (no association BTW).  Hey, it's free.  Anyway, best of luck in your quest. 

As always, with questions like this you get as many recommendations  as the number of folks responding and mostly personal favorites. I don't think you'll hear first hand experience from someone who has tried different preamps with those particular amps but I can fairly confidently tell you if you mostly listen to jazz, blues, or music with female/male vocals, you'll be impressed if you put a good quality tube preamp in front of the ML amplifiers, or any high quality SS amplifier. This is of course if you're not looking for an absolute neutral sound which most tube preamps will to deliver. If musical "truth" is important to you, a high quality SS preamp is probably a better option. Just my 2 cents.

Kalali,  Yes I am looking for a neutral true to the music sound. Why I like the Revels so much. Thanks for your feedback. 

Soix is right on with the Pass Labs x2.5. It's a great preamp and Pass Labs is second to none where customer service is concerned. Joe