Time to add more bass

[chevymotocross]

I'm currently running a Kicker 10'' TC10 truck sub powered by a Sony Xplod 4 channel 600w amp and a pioneer headunit. I have my gain and low level boost turned all the way up on the amp. The sub is ok, but it just doesn't hit hard enough. I'm not sure what to upgrade, or if I should upgrade both the amp and sub. I really don't need a 4 channel amp, since I'm only using the amp to power the sub (I have it bridged). The kicker box is nice but it's just that one 10'' sub. So do I keep the sub, and got a mono or 2 channel amp, or, do I keep the amp, and buy a 12''? Or, do I upgrade both?