Advcie on speakers/stereo for a 90 HT

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I have just got a 90 County Hard Top and it has 2 front, 2 raer speakers and a nice CD/stereo.

Problem is the speakers are not very good and so sound quality is rubbish as you drive at higher speeds - as you ahve to turn the stereo up!

I wondered if anyone out there could advise on a set of decent speakers to put in the 90 that would offer better sound at higher volumes.

Thanks in advance - Rich