Noise cancellation of COMMON MODE noise happens wherever the phases of the signal are compared. If the phases are compared at the inputs, noise cancellation happens there. If the phases are compared across the drivers that is where the noise cancellation happens.
The cool thing with amps like this is you get noise cancellation regardless of whether you use the SE or balanced outputs. With 4-boards in one box amps (icky) you only get noise cancelation when running balanced. 4boards-1box amps also amplify the noise through the whole amp, gobbling up overhead - if you get rid of the noise early (in either SE or 2ch balanced amps) you can improve performance greatly.
A 2-channel balanced buffer would be infinitely better than 4-boards in one box. Separate is not equal.