Speakers
There are many types, shapes and makers of AV speakers. Many makers of AV speakers set out to confuse and make you think you're getting something better than you really are. The most common example of this, is measuring how good a speaker is by how many watts it does. Even this isnt very clear because they can measure watts different ways: adding all speakers up, peak power of one speaker, RMS or other werid ways. RMS is the best one to go with (it's a more real-life measurement). This still doesn't tell you the 'quality' of the sound.
There is no real measurement of sound quality apart from your ears. My advice is to stay with brands you 'heard' of and remember power output does not specify sound quality, normally price does.
If you can afford the extra money, it's well worth getting away from "5.1 AV speaker kits" (£200 or less for all 6 speakers) and into the better range of speakers (£100+ a speaker) with a AV receiver.

"A 5.1 Speaker kit"
