Not all information you mention here is correct.
You might have a bad DC-DC adapter, because my newest HDPlex doesn't have any coilwhine. I've been in contact with HDPlex before when I had a GTX970 that caused the DC-DC Adapter to product coil whine.
But they took the feedback to heart, got a GTX970 for testing and made a revision. In my case the new revision had no coil whine with the GTX970 and not with the R9 Nano.
There were others with similar problems, but after the revision this has been greatly reduced.
Note I'm also only using the HDPlex 250W to power the R9 Nano.
Secondly the HDPlex 250W is suited to power the R9 Nano, however most AC-DC Bricks aren't. What we have asked Pico-Box to look into is to try and solve the problem that we have with AC-DC bricks with the new DC-DC adapter.
The R9 Nano has a 175W TDP, meaning it doesn't consume 175W but is designed for a 175W heatload.
It has a very sporadic energy demand, able to peak up to 400W. a regular AC-DC brick is NOT designed to do that, they are meant for semi stable loads, not loads that go from 400W to 100W in under a second.
If you are looking for my testing results have a look here:
https://smallformfactor.net/forum/threads/kees-kogelmier-a-tiny-powerhouse.279/page-4
