help choosing sub

my mtx thunder 8000 sounds really nice i dont no what kind of amp that is you have but the thunder 8000 is 400watts rms and 800watts max

You have tons of options available to you when you look outside of where you are. With the small amp that you have a basic sub would be your best bet. The risk that you take in underpowering most subs is that you will clip the amplifier and blow your sub. ( sss - smelly speaker syndrome). Make sure that with whatever sub and box you choose, that your gain on the amp is set properly. I am not a big fan of used subs unless they have a warranty on them and as much as I like Eclipse, a set of used 8's with a one year orignal owner warranty from date of purchase is not in your best interest. Nor is the nice PG sub that you are looking at, unless you are going to put a larger amp behind it to power it at a later date. Keep it simple and basic with your sub, if the basic MTX is what your budget can afford then go for it, make sure the box is also a good match to the sub.Top Secret, I can tell you but then my wife will kill me.

Clipping  = running the amp out of clean power and causing distortion that is faithfully reproduced by the speakers. A speaker does not distort all on it's own, something must cause it to. Refer to the redline on your tachometer, that redline is the point of clipping, stay below redline and things are good, go above it and things be bad. What you need to do is properly set the redline, this is done with the gain control. Remember that you have a small amplifier, it will give you some modest output at best, if you want it louder, then a much larger amplifier would be best, like the one I recommended to you earlier. You can underpower a sub all day long and it will play fine, what it will not do for you is play with force, play to it's full capability and depth and play loud. Again you run the risk of damaging any sub with an improperly set gain control and it most common with amps that are too small trying to do something that is beyond their capabilities. I have to head out to the other store now, if you have more questions I'm sure that Dyohn or Aeux will spark up with some more info if you need it.Top Secret, I can tell you but then my wife will kill me.

go with either the mtx 6000 or mtx 4500, the 4500 will be better matched to your amp, but the 6000 will sound good as well, whichever is cheaper, go with that, i think you will be satisfied