What is the best power cord for pass labs xa100.5?

Pass Labs would probaly suggest the one they shipped with amp. I assume they would know best, they made the amp.

On the other hand, does Pass want to increase the costs of their amps by including an expensive ultra high quality cord, knowing the owner will quite possibly change the cord anyway...

Pray, On the other other hand, why would a person want to change out an 'expensive ultra high quality cord', that came with the ultra high quality expensive amp? A cord chosen by the engineers that designed the amp.

Different preferences?

I think its a bit of both, I've heard power cables make bigger differences on some amps than others. If the power supply in the amp is properly designed then pc's should make less difference. Some of the top ss amps now have power factor correction built in as well. IME a lot of the boutique power cables are detrimental to sound due to poor design, they seem to be built by marketeers rather than engineers. This is really what audiophiles deserve though - look at some of the recent post/questions - power cables that will speed my power amp up, power cables that will give me more bottom end. The number of posters here looking for cables to fix a system issue is unbelievable.
The amp that reacted least to pc changes was my Rowland monos and some Airtight ATM3 monos.