New expensive power cord for amp and no change in sound?

I felt my X250.8 and XA160.8 Mono Blocks sounded better with Cerious Matrix power cords than the included ones...I do find that power cords are more unpredictable than other cables and cords...Pass does include good power cord, and I use them in secondary systems...though Pass has a reputation for not recommending upscale cords and cables, many articles say they use expensive Silent Source cables and cords in their listening rooms and in shows...in an interview, Nelson Pass said he very much liked a pair of speaker cables that Ray Kimber loaned him, until he found out they were $54,000 a pair...

Two years ago I bought a Less Loss power cord and filter which was about $1800. I put it on my equipment (one piece at a time). Pass Int 60; Innuos Zenith MK 3; Ayre  QB 9 Twenty. Don’t remember if I put it on the PhoenixUSB or not. The only discernible sq difference was on the DAC and it was significant. I’m not a reviewer with good adjectives but, I the clarity and bass seemed better and worth the price paid.

You're one of the few who are not dealing with expectation bias and realize there is no objective difference between power cables and I applaud you.

I really wish all of us would try before you buy. I understand that there is not always a brick and mortar audio store near you but there are several places like The Cable co, Music Direct, and ESP power cords that will let you return the item within a time period. It is usually true that you should start upstream first. That being said. If a power cord made no difference over the stock (black test power cord) made no difference then something is wrong both with the amp and the system. I have done a ton of testing of power cords on several systems and specifically certain components. They all make a difference over the stock power cord. The question is was it good or bad according to you. I have tested several Pass amps in my system along with several other brands. The power cord always, and I mean always made a difference. From a basic AQ NRZ to high end Kubala Sosna, Nordost, ESP, and several others old and new. If it didn’t please tell us your complete system and the brand of the power cord so others won’t make this mistake or can disprove your findings. Also, As a side note. Did you break in the  power cord. Some only need about a week or two and others need about a month. A great trick is to buy the c13 adapter to turn your power cord into an extension cord. Then use it as an extension cord for your old desktop computer, DVR, or your refrigerator which all use a lot of power. After a week or two you can finish off the final break in on your audio system. 

Think about it.  Power cords make a difference if:

(1) they are too high gauge or have cheap insulating materials or are otherwise garbage;

(2) you have a device that is spitting out interference that is picked up by the cord; or

(3) the device itself is spitting out interference and using the cord as a transmitting antenna that is picked up by other equipment.

Assuming you're not using a WalMart cord to begin with, you probably have enough space between your wiring or have an amp that is not transmitting junk. So an over-protected cord is not needed.

With one exception (the phono cord), I have found that careful wire routing (leaving space, have wires cross at 90 degrees) and using balanced interconnects eliminates interference.

(The phono cord being the exception due to the weakness of its signal and position at the very top of the chain, making it extremely vulnerable.)

And, yes, I have a very resolving system and good ears.