Should i stuff my subs?

i have had 2 mtx 8000 10" for about 6 months and i bought a new car and had them install 2 mtx 8000 12". they are in a sealed inclosure and when i really crank it up i realized they will jump on long beats and i am worried when i listen to faster music they will have some problems that the 10" didnt have. the box is just a universal and i have heard that putting cotton or other filler in the box will help with sound clairity and smoother fast beats. how much whould i put in and what material? the box i belive is 1.3 cubic feet per side and they are seperated. the amps call for a 1 to 2 cubit foot space. i am powering them with an mtx 81000d mono amp at 4 ohms 750RMS

thanks so much!

No need to add filler to those enclosures if they are 1.3cuft.  Are they sealed?  Make sure they are completely sealed.  Filler will make the subs act as if the enclosure was larger.

I don't understand what you mean by "they will jump"?

what i mean is when i crank it up and i put on a bass noe that is like 1-2 seconds long it will bump a few times instead of one long sound. like it is getting a signal for 3 notes

Running each woofer at 4 ohms wired to the amp like that actually puts them in parallel and the amp is running at a 2 ohm load, just so you know.  There are really not 2 seperate outputs, there is one mono output with two connection points.  But, you should be OK with this setup.  Yes, definately set your gain properly and then see what happens.

Cheers!

David