At some point I managed to blow the tweeters in my speakers - I have no idea when but this explained why they had sounded crap for so long - I was convinced it was just because the speakers themselves weren't very good. However, I measured the frequency response of the them to correct it with an EQ and found that the treble was really quite shocking (I had disguised it for some time with a treble tone control turned up full). This hinted to me that they may actually be broken. To test my hypothesis (as all good scientists should) I opened up the speakers, disconneced them and rebuilt the speakers. The response was identical. So, I got the tweeters replaced. Anyway, here is the super accurate response of the bass cone of the speakers which responds quite well up to about 3kHz.
Since I was left for a week with no tweeters I decided to improve the high frequency response by opening up the speakers and bypassing the passive crossover - thereby feeding all of the treble into the bass speakers - this helped significantly.
The response was measured using a Sure SM57 microphone with the microphones response subtracted out afterwards. I used a test CD with 31 pink noise bands and measured the input level with my soundcard.
The more observant among you may notice the abrupt plummet at about 5kHz which was a clear indication that the speakers were 'non-functional'. The speakers are claimed to be flat to 3dB between 70Hz and 2kHz which given the evidence is perhaps a little optimistic.