From the Pass Labs XA Owners Manual:
Interconnects and Speaker Cables
We have a general recommendation about interconnects, and speaker cables: They should cost less than the amplifier, and contain at least some conductive material. We have tried a lot of products and most work well, but as a practical matter we cannot make blanket recommendations.
The amplifier is not sensitive to source interconnects. It is also not sensitive to radio frequency pickup, which allows some flexibility in choosing source interconnects without shields, though shields are usually a very good idea. For long runs balanced cables are highly recommended for their inherent rejection of noise.
We prefer speaker cables that are short and stout. Oxygen-free copper and silver are the suggested materials. If you find any really exceptional cable made of gold, please gift us a couple hundred feet.
Fortunately this amplifier is not sensitive to the capacitive/inductive character of some of the specialty speaker cables, so feel free to experiment.
We have found that about 90 per cent of bad sounding cables are really bad sounding connections, and we recommend that attention be paid to cleanliness of electrical contact surfaces and proper connector fit.