Well, let's say there are many things to consider>
- frequency, or the frequency range of the sound they can produce - arguably: the larger, the better.
- accuracy or better to say 'colour of the sound' /aka, the speakers will play different frequencies at different volume/ - the ideal is no colour /everything plays at equal volume/ - a flat line across the whole frequency spectrum - which is impossible to achieve... At least for now.
- materials - the speaker will try to flex at a certain frequency, which is unwanted as it distorts the sound... So better materials are needed to circumvent that...
- crossover chip - which dictates which sound will be played by which driver - bass to woofer, etc... A good speaker set has the chip perfectly matched to the drivers...
And a good mix of all that creates a good speaker set. Kinda like magic, kinda not
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Well, let's say it's my favourite 2.1 speaker setup for about $100. Also there are many positive reviews about that speaker set, f.e. you can go here>
http/www.head-fi.org/products/swan-m10/reviews
Bottom line>
Buying the speakers is much like buying a monitor, keyboard or a mouse - you have to listen to them for a while and decide if they are worth it or not. It's mostly because everyone has different 'hearing' and everyone has different preferences - one likes more bass, one likes clear sound...
PS: You know - these types of speakers will force you to consider a decent sound card, because as accurate as they are, they will point out all the faults of integrated on-board audio, and they will be very brutal about it