what is happening is that you are overloading your amp when turned up to "YOUR" optimum listening level. you should have at least a 4awg - 2awg power wire if your amp has (3) 40 amp fuses.

more than likely, when the deep bass hits, when turned up loud, the amp cuts out, Right? if so, you need to shorten the ground wire, and step up in size.

Remember This.....
you always want the ground wire the same size (or larger than your power wire) and as short as possible. the "used" energy needs to get out of the amp and back to earth as quick as possible, before the amp recieves another "load." Or else it will overload and cut out!

if all else fails, you might need to "up" your charging system w/ a better alternator. but i seriously doubt that!!!!!

are you running both subs on one channel (Bridged?). if so, and all the +'s are running together and all the -'s are running together your amp is seeing a 1 ohm load!.......each v.c. is rated at 4 ohms.
so....
speaker A ....(2)4ohm v.c. wired in parallel = 2ohm load.
speaker B ....(2)4ohm v.c. wired in parallel = 2ohm load.
now, wire them in parallel, bridged to your amp and that creates a 1ohm load, and that is what your amp is seeing. which could be the problem.

i was pushing 3 12w6's(dvc) w/ a infinity beta 1000w amp. basically 800w rms total and hitting 158db.

now that was loud!!!!
HTH
mike