Thanks Brian & Iain,
Regarding the bypassing of the resistors, I did see this article some time ago. I think it may have referred to the GM500 rather than the 500D. Anyway, I did try but it gave me no improvement.
Iain, you may be right and I will probably have to revert to the Peco motors. However, as I pointed out in my correspondence to GM, it is disappointing to release two seemingly related products which are not compatible and not mention this anywhere in the data about these products.