Are manufacturer AC cables good enough?

The power cable included with your amp if perfectly adequate for your amp to work well.  High end amp manufacturers accept the market forces that dictate having detachable power cords.  From an engineering and cost perspective it is better to have a non-detachable power cord.  One less interface for those electrons to deal with and the connectors are usually the most expensive part of a power cord.  Additionally, many amp manufacturers simply don't buy into the power cord thing.  The might not say it publicly, but they think it's a non-issue.

As long as you're willing to settle for, "good enough".............

it's often interesting to see what power cords equipment manufacturers use in their in-house listening rooms...

@millercarbon " Power cords are included as a courtesy, just like patch cords. You aren't meant to actually use them. They are meant to stay wrapped in the plastic bag in the box to be passed along with the owner's manual to the next guy."

Excellent! True. I got quite a laugh at that line.