Dual Box advantage
The dual box units are sonically superior to single box designs because they have
Better macro isolation (from the outside world)
Allows even more mechanical isolation to be done to the individual transformer
Better micro isolation (from transformer right to transformer left) ,
When the transformer is fed a music signal the transformer
It would vibrate/energize proportionate to the magnetic field cause by the music signal.
This would effect each transformer performance as they pickup each others micro vibration.
The dual box cuts this vibration to zero as both transformers are totally isolate from each other
No electro magnetic interference between left and right transformer
When there is music the transformers radiates a magnetic field and this magnetic field can effect each transformer.
Keeping them separate will reduces this effect
Because of the proper isolation between both transformers the sound would improve in the areas of imaging, channel separation, soundstaging and depth and better portrayal of nuances
Dual Mono Advantage
When you have a dedicated volume control you have your balanced control as you have individual control of volume control
The separate volume control also offer better performance in the areas
Sound staging
Channel separation
Imaging
Depth
Transparency
The main reason why this can happen is because when the signals are join at the volume switch, crosstalk takes place in the switch internals
Having them separate all together would address the issues of any crosstalk between left and right channel and keeping them minimal.
Because of individual volume controls, the wires from the left transformer would not go any where near the right transformer thus reducing crosstalk via wire propagation
Hence the dual volume is a step up from the single volume control because of the crosstalk issues