Generally yes it is harder on the amplifier as higher current output is required with lower impedance loads...and current is what creates heat...and heat is bad.
Here is an example; The PG Xenon 100.4 outputs relatively the same power into 4ohm, 2ohm aswell as 1ohm....but you can see how efficiency decreases substantially as the load decreases.
http://carsound.com/articles/publish/article_413.shtml
That said; If the amplifier is rated to handle the lower impedance it shouldn't put a significant enough strain on the amplifier that it causes it to fail. It was designed to handle those lower impedances.
No.
Impedance has nothing to do with sound quality.