Power's back on, woooooooooo.
Okay, so I have two problems:
 If I attempt to 'send' packets 'from' an IP address that my computer actually has, the host OS's TCP stack sends a RST packet when it sees a SYN/ACK response to my 'sent' SYN packet.
 If I attempt to 'send' packets 'from' a different IP address, the packets get bounced back and forth several times between my computer and the router. For whatever reason, the listening server ('nc -l <port>') doesn't respond at all.