smokin box!

Okay, so I installed a complete system in this customers explorer (alarm,deck, 4ch, 4 spk, sub amp and sub).  The amps were the alpine pdx 4 ch and 1000 watt mono, the sub was a 12 inch type x.  I ran a 4 gauge into a d-block and eight gauge out to each amp.  I figured since the eight gauge to the sub amp was only about 2 feet, then it could handle it.  Well, it turns out that for the past few months he has been blowing fuses if he turned the deck up past about 85% volume loud and adjusted the level on the sub output beyond -2. So he comes in today teling me that so i replaced the eight gauge with a 4 gauge and it immediately blew the four 20 amp fuses on the mono amp.  Another installer swore that the alpine rep told him that they were having problems with these 20 amp fuses blowing alot so they suggested to just replace them with 30.  I was very much against this as I have always known to replace them with the identical size fuse.  I tried it anyway and they didn't blow, but after a minute of playing with volume at 85% and the sub level at +2, we started smelling a burning smell.  A minute or two later, smoke was coming out the vents.  Now, it would seem to me that the voice coil has fried or atleast been charred.  I pulled the sub out and each vc read at about 3.6 so they are still okay and in tact.  The guy that built the box built it about .6 cu ft too big and I noticed a considerable amount of sawdust inside the box and on the sub and the ports he put in are longer than reccomended for that size enclosure.  I am trying to figure out how this happenned when its obviously not being overpowered and wasn't running for a long time.  I checked my gain settings just to be sure, its set to about 4 volts and the deck has a 4 volt output so thats good, the subsonic frequency is set to 30 hz.  As far as I can tell, everything is good install and settings wise, I am just wondering about the box and the equipment.  

Are you sure the gain was set correctly and the amp was not sending a clipped signal?? Check with an oscilliscope and DMM???

So I talked to alpine, they said that they would never reccommend using bigger fuses than what came on the amp and that I just need to upgrade to a 2 gauge wire instead of 4 and I should stop blowing fuses.  As for the sub, he said that since the box is too big, the sub was trying to push more air than it could which cause the vc to heat up and melt.  That all makes sense, but since i measured 3.6 ohms on each vc does that mean the sub is okay?

did you build this box? or did the customer build it? didnt he have enought common sense to get all of the sawdust out and make the port the right length?