Did you increase the size of the box at all when you added the other sub? Is this a dual sub box, or was it just made for one, and you put one more into it? Having the proper amount of airspace is essential for good bass. Sealed boxes are much more forgiving on being at the exact amount of airspace, over ported boxes. Also, be sure you didnt mix up the pos and neg wires at your subs..

Sealed boxes have whats called 12db/octave rolloff frequency. This means, for ever octave of a frequency that goes down, it gets 12 decibels quieter.

Ported boxes have a 24 db/octave rolloff frequency. So this means that the ported box will get quiter, quicker, as the bass frequency gets lower. This means that a ported box can be very loud and boomy, then, when a much lower bass frequency hits, it can be much quieter.

Sealed boxes dont neccessarily go lower than ported boxes. They do however stay louder as the frequency start to get lower. Its all in a ported box's port design(length, and diameter), that will effect how low it will go, and at what frequency it will start that 24db/ octave rolloff. But the tradeoff of that, is the lower you design your rolloff for a ported box, the bigger the box needs to be, and the longer the port length needs to be. I've used a computer box building program to make several ported boxes that sounded really, really well. But, sealed boxes IMO, will always sound better, and cleaner. A lot depends on what brand of sub it is too. Some sound better in a sealed box, and some in a ported box.

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