It looks like those are new old stock from 2006-ish so that's why they're fairly inexpensive. Mark Anthony is suppose to be a higher-end brand, I have no personal l experience with them though.
I had the 800.4 before, can't complain for the price. Good amps if you can find them for relatively cheap. Though I believe the MK series was made by Concept since they got bought out. They were more higher end before they went out of business