No they aren't amazingly loud. I wouldn't expect that to be a good indicator of any kind of quality. I can go to a bar and listen to loud speakers, doesn't mean they sound any good though.
Its all about low distortion, dynamics, low level detail, bass extension and all those other words used to describe natural and realistic sound.
Maybe because your not an audio geek its difficult to imagine what £300 buys in terms of a driver but something you will relate to is graphics cards. Its pretty much the same with a high end card; you pay more for the extra performance. So when comparing a £250 card to a £100 card its easy to see what the extra money buys. The same applies for a loudspeaker driver.