Is the moscode 300 the Best $600 used amp?

Frogman, I don't know what your problem is. The OP did not ask for a comparison of a B&K ST140 and a Moscode 600 nor did the OP ask for a description of the sound of those amplifiers. The OP wants to know if the Moscode 300 is one of the best $600 amplifiers and I commented early in this thread in my opinion it is.

just my 2c to add my POV, I owned a Moscode 300 for many years, driving Audiostatic 300 electrostatic speakers with ease and control. The ONLY reason I sold it was to buy a Cello Encore double mono 50W amplifier. And while the Cello is far more transparent than the Moscode, I sometimes still regret the glooming orange lights coming from inside the Moscode. But I don't regret having the Cello. ;)

Many years ago a friend of mine who also owned a small HI-Fi shop got one of the first Moscode 300's. It was the biggest piece of junk he ever owned not to mention noisy and unstable.