ampeg cabs underpower heads speakers

Hey everyone, I've been having a tough time understanding how cabs and heads work together and wanted to get some opinions on what to buy next. My current setup is an Ampeg PF500 running into the Portaflex 410HLF. The head is rated at 500w at 4ohms and 300w at 8ohms. The cab is 800w 8ohms. (So...