Ahhhh... That feels good

"X" is a company that was the first to introduce a concept.

Later the concept was intricately linked with the name of the company itself.

It was started by seven brothers whose family name was "X".

One of the brothers is known to make aircraft propellors.

They created a company called "X" brothers.

Their product changed over the years from air craft propellors to something popular that is incorporated in many relaxation centers everywhere.

What is "X"?