Use the best one from your source to the pre.
what do you mean by "not of the same quality" ??
if one cable is better than another, in what way ??
can't answer this question without the facts.
without a criterion, cables are different, not better than each other.
i have reviewed many cables and while i hear differences, i would not say one brand is better than another.
perhaps you prefer one to another. why ?
i will give a general perspective:
consider the interaction between cable and component.
as you said, it is easy to test the cables.
just use them between dig. source and pre and pre and amp.
there are only 2 possibilities. one will be preferable.
If you can hear an audible difference between one configuration and the other then this will be an important health check of your system! An audible difference almost certainly means you have a problem either with your system or one of the interconnects is faulty.
Well designed, well matched and properly functioning audio equipment should not change audibly with different interconnects.