Different subs play differently. To the RL nature of the a speaker circuit (Resistive / Inductive), a speaker's phase changes depending on the frequency it plays. Although it may be close between different speakers, they'll never be 100% completely in phase with each other, so they'll be canceling each other out at certain frequencies, others they'll be in sync, so overall, it's going to end up sounding like poo...

 

Also depends on the efficiency of the sub. Two subs seeing 500W each aren't going to see a theorhetical +3dB if the same size 1000W sub is 3dB more efficient. My friends' setup is the worst - old gen. S10 with two different subs/amps/bandpass boxes, and it sounds so bad.

 

Basically more technical way of saying what posted. Well said..

 

I personally have a 1000 watt 2 ohm in a single box and two 4 ohm 250 watt in a dual box all wired at 1 ohm into a 2000 watt amp and that seems to work just fine but when i had the 250 and the 1000 in the same box it didn't sound good at all, my advise is if you want to run a 300 and 350 don't do it in a dual box, buy two single boxes or just sell one and buy matching subs/single sub