Amp or Sub cutting out

I just finished installing all of my stuff last weekend I have an 12" Alpine type-r (300W rms) and a Kenwood eXcelon amp: http://www.sonicelectronix.com/item_8651_Kenwood+eXcelon+KAC-X10D.html

Everything seemed to be working fine when I set it up but then I realized that the sub was turning off at low volumes. I played with the settings on my headunit and the amp for a while and got everything working but now it cuts out when I turn it up too loud and the sub won't get as loud as it did before. I just pulled it all out today because I needed to carpet my box so I am going to mess around with it tomorrow but does anyone have any ideas.

This is my headunit:
http://www.crutchfield.com/S-pgNFDT...n.aspx?i=130DEHP590&tp=5684&tab=detailed_info

I ran a 4 gauge power wire and an 8 gauge ground wire. The RCA cables were from my last setup but they were cut off on the headunit side so I had my brother solder on replacement tips that we got at an electronics store. From the amp to the sub I have 18 gauge speaker wire and the sub is wired in parallel with the same speaker wire. It has dual 4ohm voice coils so the load is at 2 ohms. I have the LPF on the headunit SW output set to 125 and the LPF on the amp was at 150. I also had the gain/input sensitivity all the way up.

Im going to try and play with the gain on the amp tomorrow and see what happens but other than that I am a little lost.

Thanks for the help I got a chance to play around with the gain on the amp the other day and it was making the sub clip since I am only running 1 sub. The amp overpowers the sub a lot so I set the gain to about half after testing it at different levels to make sure that it couldn't cause the sub to clip even at high volumes. It sounds good now and works great but I haven't ever heard a sub clip before so I didn't know what was going on before.

My last sub did almost the same thing before after I had it for a while but it ended up being because one of the tinsel leads broke.

I got a Kenwood eXcellon mono it is 600w at 2 ohms and I am running it to a 12" type R with dual 4 Ohm voice coils. I have it hooked up in parallel so it is at 2 ohms. When I bought the amp I thought this was the newer model type R which handles 500w rms but it is the older one that can handle 300w rms. So it was definitely the amp is overdriving the sub I just needed to adjust the gain so that it wouldn't clip. I did check the voltage though and it is fine since this amp has so much power I think I might have to upgrade to a more powerful sub in the future but for now it sounds good and hits hard so im not too worried about upgrading the sub.

It's a really nice amp and it is stable at 1 ohm with over 600W RMS so it should be cool if I add another sub and wire it at 1 ohm.