Sounds like the tweeters are blown. Very common fault usually caused by running too loud. Easily tested by your dealer. Tweeters should have a resistance of a few ohms, faulty ones are either open or closed circuit (I have seen both).
Believe it or not, many users don't realise and simply wind the treble up and the bass down!!
Replacement is the only option and should not break the bank. £50 for parts plus labour probably.