If you're running two 8 ohm cabs from your head, the total impedance is 4 ohms. If your head is 300w at 4 ohms you'll be ok . The only thing you'll have to worry about is the power handling of your cabs. But from what you've got at the minute it's all hunky dory!!
I've had an Ashdown mag300 head and run it into two 8 ohm cabs and personally it sounded great.