I've just noticed this 'long' thread. It is of interest to me as I had
a not dissimilar problem. The use of superposition is absolutely
correct in a linear system where the system component values do not
vary with the signal applies. However, when applying the ac component
the skin effect phenomenon will deplete the current carrying carriers
from the centre of the conductive component thus reducing the effective
cross sectional area......which will also effect the dc
resistance....or will it?

Danny