High quality speakers come down to components and specifications, just because they are "high quality" doesn't mean they sound good... Everyone's ears are different and they like different sounds... So sound quality is all a perception of how you like sounds.. Some people think $20 speakers sound great and cant tell the difference or care for the $2000 speakers. This is because their ears cant tell the difference, doesnt mean there isnt an actual component difference.
Basically the human ear can here 20hz - 25khz (most people) So ideally you want speakers that can fill that whole range of frequency, this is called frequency response on speakers. The more sounds you can hear, the better the speaker is "in general"
Most speakers now adays are built at 8ohm specifications, some are 4, some are 6, etc etc... This number doesnt mean a whole lot about build quality, it really means more about the amount of impedence that the speakers can handle, this relates more to power than anything. IE: The higher the number of ohms in impedance, the less wattage required to drive them. Some people think this means higher quality speakers because it requires less power, this isnt necessarily true, so dont get hung up on that.
Effeciency: This basically relates to power and how cleanly the speakers can use the power. Most speakers are around 85db @1watt and 1meter. The higher the number of dB per 1watt and 1 meter, the more effecient the speaker uses the power its given. So basically it requires less power to turn the speaker up louder with the same wattage...
Ok, now that thats covered, you get into a whole gammat of speaker designs, materials used etc. A lot of this is all subjective due to the persons ears...
Some people like the sound of horn tweeters, some like the sound of EMIT tweeters, some like polydome etc. I personally believe the material used and the quality of the build depend on the manufacturer and how long you want the speakers to last. Some speakers sound great, but the material for the tweeter breaks down quickly, some speakers sound ok, but they will last forever. I will let someone else chime in that knows more about this topic than me.
For Headphones for the price range... Look at the ones below.
Audio Technica M50 Headphones.