Preamp under $2,000.

If interested in a tube based pre I would check into Quicksilver or Aric Audio.

I am an MF Dealer.  The older M6 is a nice unit.  It was replaced with an M6s Pre that retails for $1890 so $1500 for an older unit is not a particularly good deal.  

It is detailed and relatively neutral.  Like most MF products, it is made in Taiwan which results in a strong value.  They have been shifting some production to the EU but I don't think this is one of the units that has moved.  For the money, it is brilliant.  

Most places will give you 10% off and free shipping on a new unit which will put you within spiting distance in terms of price on the used unit.  

The other unit I would look at seriously if you can find one is the Ayre K-5xeMP.  You should be able to get this For $1700 to $1900 used and it is worth every nickle.  I used one for years and loved it.  Probably the item I regret selling the most.    

To OP: I would recommend that you take a look at Doge Audio 8 Tube Preamp: https://doge.audio/product/doge-8-clarity-2019-edition/.

I have been using one for over five years now and extremely happy with it. I use it with a pair of Doge Audio 9 Mono Power Amp to drive the Magnepan 1.7i. I also pair it with a set of Manley Snapper Mono Power to drive the Tannoy Turnberry. It's not a very popular tube preamp here in Audiogon, but it has such high performance, not to mention the value ($1,500), that there are cult follows with this preamp.