I am not sure if that statement is true. I am sure there is some dispersion and non-linearity in internal impedances which would make it a function of current.
im not sure if it would be linear either. BUT, i do know that in a decent amp, its SMALL. it is big enough to be a factor. but, i would bet that with different loads, its doesnt very a ton. and given that its a small factor already, and the fact it doesnt vary a whole lot, it might come out to be a negligable factor.