The Celestion 70/80's are in the amp. They are not the best speaker. Which speaker would be a good substitute?
Almost any WGS models would be an upgrade :). Check out my blog on mixing speakers. My choice would be either the ReaperHP/ET65 combo or the Retro30/ET65 combo. Either will handle the 150 solid-state watts of the Mustang IV just fine, and the tone should thoroughly rock your world - it'll be like a whole new amp!