what alpine hu are you using? it may have 2 volt preouts. 4 volt will also help with this problem. and keep the crossovers set for front and rear at about 125 with a slope of 18 db/oct. it's a steep slope that will really keep a lot of bass out of the stock speakers and will help a lot with distortion. in my truck i left the rear speakers stock and did exactly that with my alpine hu settings (cda-9855). i can get the hu up to about 17 or 18, which is where the internal amp on alpines tends to start clipping with the crossovers set. also, do what was previously stated, have the amp output the rated 750 rms at about 17 or 18 on the hu. also, although the subs are rated at 400 rms each for a total of 800 rms, you're putting them in a ported box. they will handle less power in a ported box. 300 rms each, look at the manual. now this is going to depend on the size of the box, however, using a prefab box, i wouldn't go more than probably 300 into them. also, make sure you are using the subsonic filter on the kicker amp. i'm not sure if this is even defeatable. the manual says it's fixed at 25hz with a 24db/oct slope. with a ported box and the power you're trying to put into these subs, if you don't use that, you're going to blow them. powered properly and tuned properly, they'll sound pretty decent. also, kicker has a prefab box that works well with these subs. it's tuned to 40hz i want to say. that would be decently loud. you should have went that route because they're prewired and you somehow turned a 4 ohm load to 2 ohms with dual voice coils (so you think). this could also be your problem. guess i could have misunderstood that one though. did you have a set of single 2 ohm subs wired into 4 or a set of dual 4's wired into 4? or did you somehow maraculously change the impedence of the coils?
