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

Depending on the manufacturer a couple of weeks may not be enough break in time.  For example, my experience with Morrow Audio cables take at least 300 hours to break in.  In my experience power cords can have a significant impact depending on the resolving power of your system. Get yourself a break in file on repeat and give it some time. 

You said “Yes my amp is a stereo Pass 250.8 and the manufacturer said don’t bother”

So, Nelson Pass then properly designed the amp and its power supply. I am not surprised you don’t hear any differences. Why not listen to one of the best engineers?

My First experience with power cords was back in the early 90's Being in the electrical field I was sceptical at the same time open minded because all wire is not created equally. The Store had the exact same system as mine, Amp speakers and CD player. And they swapped out a $350 Nordost Blue haven cord and I could hear a difference in the store. They gave me the power cable to try in my home and I was shocked to hear the difference. I was not really into audiophile if you will, just enjoyed music at that time. the components were in the wrong part of the room and no treatment, due to being married and she had top have the house look a certain way. Blah Blah Blah.

Fast forward To new upgraded audio gear, and the Wireworld dealer gave me 3 power cords to take home. After a week of listening trying all three, I chose the one in the middle at that time it suited the room and the gear I had. This is where my cable obsession took off. As well the room got re-arranged, and the gear found its new home in the room. Room treatment started as well, and then I upgraded all the cables to Silver eclipse 8 and never regretted it for a moment. I found that once you hit a certain price point in Gear, there is a noticeable difference in cables.

The improvements are all in your head . You spend a lot and expect a lot. Power is    power. In M.O. any decent manufacturer of good equipment would consider a minimal investment in power cords good business. This would be easy and inexpensive way to improve their sound quality. While I've seen sound improvements in speaker and interconnect wires without spending Ks. I don't believe a heavier power cable would make much of a difference. Look more to overloaded outlets change 15amp to 20amp circuit breakers Of course this is Only M.O.