220 Pro Ci vs 250 expert

[Confused]

For fun I've just compared the 220 Pro against the 250 Pro.

I expected not to be able to hear any difference as the electronics are the same, the firmware is the same, the settings were the same, the case is different, and the software limits the power at high levels in the case of the 220. I listened at levels of couple of watts.

I am perhaps being slightly pedantic here, but in fact the electronics in the 220 Pro and 250 Pro may not be exactly identical. It was true that the innards of the Expert 200 and 250 were indeed absolutely identical, but there is a difference between the DAMP board in the 220 Pro versus the 250 Pro. This might simply be related to the method used for cooling with the 250's different case, but as the board designations are different it is quite possible that there are differences in the actual electronics also. Quite what this means for the sound quality that your ears are ultimately detecting is impossible to say, but it does hint that there might be a hardware related reason, or it is at least a possibility. 

This was mentioned a while back in the thread linked below:

https://devialetchat.com/Thread-D220-Pro...the-inside

Another factor is that the firmware for the PSU is set up slightly differently between the 220 Pro and 250 Pro, with respect to the two unit's different cooling capacity, but as you mention, it is hard to fathom why this might make a difference if you are only using a couple of watts.