Is the moscode 300 the Best $600 used amp?

If you're going that route,ensure you get complete disclosure from the seller regarding transformer hum.Common issue with 300's.

I have a B&K ST202 amp. Is the moscode an upgrade to my amp??

+++I have a B&K ST202 amp. Is the moscode an upgrade to my amp??+++

IMO, yes. A Moscode 600 replaced a B&K ST140 years ago. It was a step up, with some of the typical differences between tube and ss coming into play; eventhough the Moscode is essentially a ss amp (with a tube driver), and the BK was considered to be "tuby" sounding. The Moscode sound will actually be more "neutral" tonaly, but not as far as texture. The moscode will have larger, more rounded images, with more dimensionality. I found the B&K sound to be drier, but with a darker hue. This, of course, in my system. I hope those descriptions help in some way. Is it the best amp for $600 with the qualities you describe. IMO, no. A Conrad Johnson MV50 comes to mind. Or, if you can stretch the budget a little more (I hate when posters say that), for me, this is the one: http://www.audiogon.com/cgi-bin/cls.pl?ampstube&1297226361&/Dynaco-Mk-Iii-(Two)-Fabulous-new-build

Good luck.

Going from a B&K ST140 to a Moscode 600 is more than a step up. How do you compare a wimpy 105 watt amplifier that falls apart into a 4 ohm load to a 300 watt amplifier that doubles down into a 4 ohm load?

Moscode 300 will be a huge step up from the B&K. To me the biggest differences will be improved dimensionality, tonality and detail (w/o being over analytical). Just my opinion. I have owned both the 300 & 600 Moscodes (stock & modded). Moscodes are a great value!