What the difference bewteen a monophonic amplifier and a dual mono one?
To improve audio quality, a Brand can choose to make an amplifier with a mono or dual mono design. The same is true for the players, preamplifiers, power amplifiers etc...
The better is the channel separation, the better will be the soundstage. So a mono amplifier will be better than a dual mono, and much more than the simpler designs.
Partially dual mono
It is just a bit better than the previous type.
One half of the amplifier is specular to the other one. There is simmetry.
When a "box" contains just one channel. Everything you can see is just for one channel. It is a common design for power amplifiers (the monophonic ones), and it is extremely rare for all the other type of products.
As stated above is valid also for preamplifiers, power amplifiers, cd players, etc etc.