Do you need two of them to work or can you just use one monoblock and listen in mono?
This will depend largely on your source of music. The majority of music media today is in stereo. So, to get most out of them you will need a pair of monoblocks, one for each channel. Having said that, there a few modern day aficionados who maintain that the purest sound can only be achieved from a dedicated mono system. That's upto to you to decide.
The MKIIIs aren't my favorite. They are ok, but not great. IMHO, the ST-35 is a sweeter sounding amp overall. A whole lot cheaper to build as well. That's just my deux cents on it. -Min