Thanks a lot!It depends on what you mean by 'full potential'
A90D can output 10W (balanced out) and 2.5W in SE mode.
The ADI2 DAC can do about 1W in 20 ohm.
So the A90D can reach higher peak levels. At 97dB/V the ADI-2 can reach 110dB peak SPL (so about 90-100dB average) and the A90D (balanced) can reach 117dB so can go quite a bit louder.
I would assume you value your hearing and don't EQ the DCA so would expect you will always remain below 1W in which case the A90D has no benefits.
But on occasional 'let's see how impressive this headphone really is' moments and you crank it up with music that has a ton of bass in it then chances are you may be clipping the RME and would not yet clip the A90D. In that case one could say the A90D may be worth the extra expense.