Reddit mentions of MDFS1215 1.5 Cu Ft - Single 12" Sealed MDF Subwoofer Enclosure

DIY. While not top tier stuff like SVS or HSU, you can stitch together a truly awesome sub for very little money these days.

https://www.amazon.com/Rockville-W12K6D4-V2-Subwoofer-Compliant/dp/B07C8KV239/ref=sr_1_7?ie=UTF8&qid=1537516086&sr=8-7&keywords=rockville+sub

Surprise surprise! This sub has been reviewed quite a bit in the car audio scene. It's quite a bit better than the price would imply.

https://www.amazon.com/Belva-12-inch-Subwoofer-Prelined-MDFS1215/dp/B00EV6ED2A/ref=sr_1_15?ie=UTF8&qid=1537516271&sr=8-15&keywords=12+sealed+subwoofer+box

Good enough for sealed. With a sealed box you will never hit the "movie" lows, so you don't have to worry about convoluted ports and whatnot. Maybe throw some bracing in if you are worried about cab resonance.

https://www.amazon.com/Behringer-NU1000-BEHRINGER-iNUKE/dp/B00LGNFALY/ref=sr_1_1?ie=UTF8&qid=1537516541&sr=8-1&keywords=inuke+nu1000dsp

Only does an actual 600 watts into a 4 ohm load bridged, but it has fairly powerful DSP and you probably don't actually need more than 600 watts peak. And it's only $200, though you might want to mod in a couple 120mm computer fans for quiet operation.

$350, plus fans, cables, bolts, and some sealant. So $400 and a bit of elbow grease, and you'd have a sub that would shake the hell out of most living rooms. You could also get a shallow mount box and sub for roughly the same price and still get big bass out of a little box;

https://www.amazon.com/dp/B016IH7XD0/ref=sspa_dk_detail_8?psc=1&pd_rd_i=B016IH7XD0

https://www.amazon.com/dp/B01BXE8ZHG/ref=twister_B01MYY0ZTM?_encoding=UTF8&psc=1

Same Behringer amp. Or you can use the savings and get something nicer, like a Crown XLS1002.

You'd get better sound quality putting a 2 way component set up front and leaving your rears stock. This is what I'd go with on your budget.

Headunit $230

Sub $60

Box $45

4 channel amp $135

Front components $120

Total is $590, so a little over your $600 budget after wiring and adapters. If you run this you would power your front components using the first 2 channels of the amp, and you'd power the sub by bridging channels 3 and 4. You'd keep your rears stock and fade your sound all the way to the front for the best sound quality. This would be a pretty nice budget sound quality system.

For the fronts you would just have to buy 6x8 to 6.5 adapters for your vehicle and mount the tweeters somewhere, usually either your doors or a-pillars.

http://www.amazon.com/1-5-Cu-Ft-Subwoofer-Enclosure/dp/B00EV6ED2A

Try that box. I know you wanna save money on this stuff but the box is honestly really worth it.