Why my system does not sound Loud

Ohh and if your VC are out of fase it can cause problems also so you might pull the subs out and verify that everything is in order

correct you are. missed how big they were sorry bout that. but 80 is still a exelent spot for most subs in general.  and to be honest if your using an outboard EQ I would only use it to adjust my crossover freq. it adds simplicty and takes out any extra processing that can cause noise or screw with the slope in wich the freq. meet.   

Shouldn't he only have 1 low pass crossover going?  Sounds like he has both the HU, AMP, and possibly EQ crossover all set at once.  I don't know all that much about it but couldn't these multiple crossover be causing some strange phase problems and an unnaturally steep cut off slope?

Yes, definately.  One crossover is all that shuld be active for a given frequency range, or all sorts of wonderous effects can occur.  Thanks, I missed that one.

Well that will definately cause a noticable drop off in SPL.    Now you need to decide why it blew.  I suspect your amplifier input gain setup could be the culprit.

But tell me somthing why only one Blew and the other one does not.?

Could be many reasons for that, including the luck of the draw.  Most likely the other one is getting ready to go as well and the one you noticed simply went first.  Could be it was defective.  Could be differences in voice coil impedences caused one to drive harder than the other.  No two pieces of gear are exactly the same.

I highly recomend you set the amplifier gain again, just to be safe.