Few answers to you question.
What causes speaker distortion? Is it because of low quality speakers
Not necsarily. You can have cheap speakers and not distort if every thing else is right. They may not sound clear or strong or maybe not last long but just becuase they are cheap does not mena they will distort.
yes it could be this too. If you amp is not sending enough of a signal then it could sound distorted
too loud sounds for that particular speaker or system?
This is usually the case. especially if they are new speakers and you tend to blast them. Check how much power your speaker can handle usually on the back side of the speaker and then check how much power your amp is putting out per channel.
One other cause,:
Broken speakers. If both are distorting then it MAY not be this but I would rule it out before trying every thing else. ( a quick way i test speakers -passive speakers or without a built in amp- is to unplug the speaker from the receiver/amp and touch each wire to a 9v battery one wire per pole. if you have a 2 way speaker -tweeter/horn and bass then you should hear two sounds a high tone click and a low tone click or thumping. if you do not hear either of these then one is broken and causing distortion) If they are broken and good speakers then you can probaly get them re-coned or just purchase a new speaker (the broken part) and replace. Most speakers can be easly replaced just by using a screwdriver.
Just read the post that was posted as i was typing and I echo the CHECK all your wiring first. and if you bought the whole set up together then disregard the the amp might be putting out more that the speakers can handle.