Bandpass, Sealed or Other?

Id go ported.

If you have the room build a transmission line style box.

Solid info from fosgate3, nice to hear good advice and like he said here is one of his mentionings. I would head in the direction of 2 subs, if they are within the amps capabilities. Before I chose any sub or box combination I would want to know about the amp and the vehicle. It sounds like you have a one note wonder box, not what you imagined from a Diamond sub. Before you run out and buy new subs, I would build (or have built) a ported spec box for that sub, lets see if we can get it operating as it was designed to.

Why would I choose two subs over 1, generally speaking they can attain a higher spl than a single comparable driver, they usually play deeper, they tend to "smooth out" the bass response over a wider range of frequencies. A one sub system in most cases for most but not all people tend to get overworked on a regular basis, a two sub system does not need to be driven near as hard for the same results (in most cases), they can play to a louder sustained volume level for a longer period of time at a lower overall gain level as compared to the single driver. Now I' not saying that you must do this, this is just from pratical experience, I have also built numerous single sub systems that absolutely leave some dual sub systems in the dust, a reference 10" MB quart sub for example and pretty much any Eclipse aluminum or titanium driver as well. What do I use, a pair of Eclipse aluminum 10" subs in a MR2, ba bam.

"I have also built numerous single sub systems that absolutely leave some dual sub systems in the dust"

You and me both Forbidden.