Well, thanks to @mrehorstdmd for the solution. I changed my M307 command to: M307 H2 A11 C99000 D2000 B1 which seemed to work. It started heating and didn't give me the error!
The unfortunate thing is that is blew my 500w power supply (whoops). That was more my error more than anything else as I should not have tried to run that power supply more than 80% of its rated wattage. Good thing I have fuses on all of the electronics so all I did was blow the power supply and not kill all of the other electronics 🙂
I'm going to put the original 300w power supply back on the main unit and get a second 450w power supply for the heaters (which is probably a bit safer anyway). The new power supply will arrive in the next couple of days and I'll post the results...