Unable to re-install Snow Leopard after upgrading to Mavericks

I upgraded to Mavericks before realizing that my USB microphone interface would no longer work with Mavericks due to the manufacturer not updating drivers... So I need to revert back to Snow Leopard. I don't have a time machine backup from snow leopard, so I erased my hard drive and attempted to re-install snow leopard from the installation CD's. But it won't work! It says that I cannot install Snow Leopard on my machine, only restore from a time machine backup.

I suspect mavericks messed with my firmware or something... How can I get the computer back to the way it was when I first bought it? It seems like you should always be able to revert to the OS that shipped with the machine, but this is proving very frustrating!