Well Stock, let's disect this.
1. You have 2 x 6" in the front, 2 x 6" x 9" in the rear. That's fine. You're running these off a 1200W Max (I'm guessing) amplifier that would no doubt be a 4 channel. This would probably be closer to around the 50W RMS mark per channel. Still fine.
2. You don't want less "boot rattle" but nor do you really want more. That's fine, noone likes that much anyway. A little sound deadening in the form of Stinger Road-Kill or it's equivalent will help sort some of that out.
3. You have an 1800W Max (again I'm assuming) amplifier that you want to run a sub from. You mentioned earlier that you were only using one channel, I assume that it is a 2 channel? What brand/model is it? If it is 1800W Max then your RMS will only be around 300-400 so no point running a 1000W RMS sub on that anyway. But if the quoted figure of 1800W encompasses 2 or even 4 channels, then that's bad news as it means each channel gets a smidgeon of the quoted power.
4. You would prefer to not adjust your box. That again is fine but however each subwoofer needs to be used in a specifically designed enclosure. Some sealed, some ported. Some need small volumes, some need large. No matter what subwoofer you buy, 1000W or a 300W, you ARE going to need to change your box. At the very least to drive the speaker the way the manufacturer intended it.
Let's start by giving a little more information on the amplifier from there people can match a subwoofer that will drive to it's optimum. This may mean that it won't be a 1000W one, but a 300W subwoofer can sound just as good as a 1000W one any day. The only real difference is you won't be winnning any SPL contests, but being in the sticks this was probably never a concern anyway.
Cheers
