Why is the D400 "better" than the D200

[Antoine]

From the marketing texts of the D400: 

(I'm afraid something got lost here in translation e.g. 'Current return from the total mass')

Finally, a dual-mono solution at an affordable price! The Devialet 400 is the most comprehensive and high-performance dual-mono model. Extreme power (400 W), an inexhaustible wealth of adjustable parameters and unsurpassed performances (THD + N 0.00025%), the Devialet 400 finally enables you to make the transition to a  system comprised of two power blocks.

In this configuration, the Devialet 400 offers you unequalled musical resolution and beats every record. Devialet's technology is magnified through its operation in mono-block mode via a dedicated power supply and an ADH core for each channel.

• the power supply and balanced converter dedicated to each speaker offer you more power and greater dynamics, most notably by way of SAM technology
  
  
• the use of the Devialet 400 in mono-block mode enables the elimination of the current return from the total mass and the further improvement of the performance of the DAC "Magic Wire" and  analogue inputs (THD + N 0,00025%). The dual-mode also reduces signal jitter for all digital inputs.
  

I think it boils down to separated power supplies and ADH cores (less crosstalk/noise/ground contamination, more power per channel) and the use of 2 DAC IC's (I think the DACs run in parallel in order to sum the outputs and average the difference) vs. 1 per channel (less distortion and jitter).