What 2 channel Amp would you Buy for $6k?

Something in the Pass Labs lineup would work very well, depending on your power requirements. I love my Xa30.8. One review says that it competes with a highly regarded SET  amp (can't remember the name offhand) that sells for over 20k, plus you get the firm low end punch that comes with SS. It's very conservatively rated at 30 watts. It's easily driving my Vandersteen Quatros. I bought mine used for just under 5k.

My speakers are Magnepan 20.7s

Amps I have had 

Mcintosh 452
Sanders Magtech
Bryston 4b3

Of all of those the Bryston is by far the best, the worst is the 452, weak base on the 20.7s and not a great sound stage.

Someone already suggested Krell Fpb350 mc. I second that, or stretch for a pair of Fpb450mcx amps. Same chassis, different driver board and newer. Both world class amps that are amazing. Enjoy....

Perfect_sound

If you’re looking for tube gear, I have a Decware Torii Jr that sounds good and recently upgraded to Audion Siver Night 300b Special Edition. I’m loving the Audion paired with Zu Audio Superfly (101db) speakers. With only 8 watts I usually only dial in 1/4 of the volume and it delivers plenty of punch, dynamics and detail.