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

There is no definitive "best" of anything. There are way too many variables involved. The "best" you can hope for is to find something that works well in your system. You can then say it's the "best" for you until you find something you think is better !!!!

Actually I didn't try to use the original power cords 'coz it doesn't look fancy so I used the cords I bought from ebay . . . thinking that it will sound better. Thanks to Rrog, the original sounds great. Nelson Pass can't go wrong.

My amp is 5 feet from the wall outlet, therefore, I found a 6 foot PC to work best.

I have the XA30.5 amp and heard minimal improvement with using a Pangea AC-9 on it. I tried others as well, also minimal improvement. I tried an Audience Powerchord "E" version and heard a significant improvement, which really surprised me.