Thermal loading help? Sundown Z v.3

Okays guys ima list the components so maybe you can pick which would be the problem as im sure it is my wire.

-Sundown Z v.3 12 Dual 1

-Alpine MRX-M100

-Audio Technix OFC 8 guage speaker wire

-Cadence 4-guage power and ground

-I don't not have a big 3

-RED top optima that is crankin 15.20 volts strait to the amp

-current box 2.1 cubes overall. 1.75 internally. tuned to 33 hz (im aware its not big enough, Im currently building a kerf ported box that is 2.8 cubes.

I also had this thermal problem with my sa-12 so I upgraded to the z v.3 and it handles it a lot better but the dustcap still gets warm after 2 or 3 pounding songs. The sub is a Dual 1 wired to 2 ohms bc alpines lowest impedence on that amp is 2 ohms. Im only turning the deck to 18 out of 35. The sub is not distorted what so ever and the amp gain is only cranked halfway and it can still bottom it out and clip it all the time on low power. I do not understand why its getting hot so quick. my only reasons are...

1) 15.20 volts

2) Crappy Cadence wire

3) Amp

4) Box size

Any help is very appreciated. Thanks