Hi,
I had the same issue... crap sound in a 1994 cab... so I changed the head to a reasonable Sony unit, changed the dash speakers to a pair of 4" Pioneers (easy job), and put a set of nice Focal 6" (I think) in the rear sides - actually very easy to remove the panels... just get something in behind on top and gently pry the panel out... a doddle. I needed to cut and bend a little piece of metal away behind to get the speakers in, but the result is a neat job and a whole lot better sound for not so much. Oh, and I routed decent cables to the back speakers aswell for good bass (and because the rear are amplified at the speaker, not from the head unit).... all for about 250 euro well spent.

Hope this helps.
B.