Preamp under $2,000 used

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.

"I am looking for a neutral true to the music sound.."

I may be in the minority here but the main reasons for getting a preamp are multiple inputs, volume control, extra gain, and perhaps more importantly, the "flavor" it could add to the music. If your objective is something true to music which is difficult to measure anyway, you should definitely audition before you buy. I also wouldn't rule out a passive preamp if you can find one to fit into your system.