The Polk MM series is a great series of woofers. I seriously always overlook them. Neo motors, decent Xmax. Liveable small box requirements.
JBL/Infinity subs are also a bargain/gem that often get overloked as far as car audio brands.
The sundown Audio SA line are absolute beasts. the 12 works well in 1 cubed sealed. they run a little higher than the others mentioned. They are designed to be low distortion. Have excellent Klippel results and really produce. They shine when you port them but they sound good sealed. Have an aquaintance who ran 4 of them sealed in a Tahoe. ridiculous output, and great tonality. He was a rock music guy and he was tickled.
Have also run tens and a single 12 ported and if you like extended lows, they will definitely pressurize your vehicle.
I push Dayton Audio subs a lot the Reference series subs (HO and HF) are great performers.
Alpine R series are excellent.
I am a huge fan of Sundown audio. They have a SAX1200D that will do 15oo watts at 1 ohm all day long.
Kenwood has some very decent sub amps. reasonable.
The best value that i have installed for kids are the Audiopipe brand of class D. the 1800 and 3000 are or seem to be solid performers for reasonable money. Plus you have some room to grow.
I bought a 1500 watt mono sub amp and haven't looked back since then 4 years ago or so.
Look at the brand outlets for bargans. I look at the Harmanaudio ebay store as well as the polk audio and find really good deals. Like these.http://www.ebay.com/itm/SINGLE-MM154...item5ae00dec20http://www.ebay.com/itm/PA1200-1-Hig...item53e6feecf3
Yes it's a 15 but what a 15... and the amp has all the power you would want plus if you ever changed setups, you can add more subs without fear. A true beast.