What's the difference? The obvious would be the quality and also the grade of parts used inside the amplifier. And most cheaper amps are prone to break down after a year or two of usage. But as for expensive amps(which are of international credit, not as in "expensive cap ayam"), the parts used usually comes with certain grades and would last years. I've got amps that are more than 10 years old and they are STILL working flawlessly, i doubt you can say the same for cheapo amps.

As to whether does it sound different or not, this is VERY much debatable,