I use twin 15" Dayton Quatro subs in a sealed box, about 3 cubic feet. They're fairly high sensitivity and you can easily get away using a 240-250 watt plate amp in all but the biggest rooms.

This is one of the most popular DIY projects on the PartsExpress forums, it's inexpensive and packs a mean punch for HT while offering excellent sound quality for music listening.
My first unit replaced a $1200 commercial sub. Much better sounding, though much above 105 dB in my room you might want a bigger amp or a 2nd unit. Not bad for $250-$300 in parts and materials, not bad at all.

There's other possibilities depending on your budget. I built a 12" Titanic sub for my parents which is also a pretty darn good unit, and a few of the guys here have gone with the 15" Titanics and are very pleased.