I've never upgraded from 550 to 750; however, I have upgraded from 750 to 870 or 1GHz. When upgrading from 750 to 870 or 1 GHz, you have to change the white or clear plug-in to red plug-in. If not changed, you will have levels bouncing all over. They have to be changed out. Its been a while, but its something you may want to look into. Good luck!!
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