The highest quality headphones you can buy have the finest sound you will ever get from any headphones. Hence the best good quality speakers that you can get into headphones have already been invented - if you've got the money.
Headphones were based off the earpiece and were the only way to listen to electrical audio signals before amplifiers were developed. They were first successfully developed in 1910 by Nathaniel Baldwin. He made them in his kitchen and sold them to the Navy.