A capacitor wont affect whether or not you blow a fuse. They simply help deliver the power to the amp exactly when it needs it. Check if the wire jacket is still good. If you went through the firewall it is possible that the jacket has been cut or worn through and the lead is grounding out before it even gets to the amp. If that's not the problem then check the sub wiring. If your subs are dual voice coils then depending on how you wired the subs you could end up running the anywhere from .5 to 16ohms. Check the wiring and make sure it matches what the amp is capable of. A simple google search will yield sub wiring diagrams. If that doesn't work, maybe consider using a larger fuse on your lead. For 800w a 100A fuse should do. Im running 2400w on a 150A fuse with 4ga lead and ground. 100A should be more than enough for you.