new 18 for my 5.4 box

[Sniper]

So...

I wanna put a new 18 in this box i've got, I need something that will hit harder but dont want to pay a ton more.

i'm gonna go in head and say maybe around 250-300 Dollars, i'm currently looking into a USAmps XT1600.4V2 so i've got about 400 RMS X 2 @ 4ohm to play with, not quite sure what that runs at 2ohm or 1ohm, or if the amp can even take a 1ohm load, so im going to be careful there.

so im gonna go out on a limb and say around 600 rms x 2 @ 2ohm (as stated, I really dont know what it puts out there)

so, I need a driver I can feed this much power into it and still get some decent sound.

-unless-

someone knows of a decent sub/amp combination that will work good with this box of 5.4 cubes, I can always add a new top layer and chop the top baffle out to recycle the box (afterall, it was only built last week)

was thinking maybe 1 18" Fi SSD, but im not really sure if I should go with an SSD or just save my cash then go with a Q.

but I am very unsure of how the Q would perform in 5.4 cubes ported @ 30 (which will probably be closer to 35Hz when that sub is dropped in, hence why I tuned it low to begin with)

I guess thats two questions in one.

TL;DR Version ===

Question 1 = What subwoofer @ 18" Diameter would you recommend for under 300$ Shipped?

Question 2 = If I were to retune this box, what would you recommend for the sub selected in question 1? (keep in mind I love the Low low bass)