Yea, i would have thought some series resistance would have been in order, or a tube amp with around 4-6 ohms output resistance or the new amp1 of NP.Maybe it might need the tube or current amp to give the proper response?
I have worked with both the 206e and the 166e and found the 166e to give a better mid and HF response , but the overall performance of the 206e makes it a slightly better performer.
ron