Bic F12 ,Polk PSW505, Klipsch rw12d, a pair of Dayton sub-1200's, or 3 dayton sub1000's... I think out of everything I have heard a pair of sub 1200's for around $230 shipped on sale is about the best sound for under $300, when the 12d is on sale for $300 its also a god buy, the psw505 is very capable for the price, and the f12 is about the best single sub budget friendly value I've yet to hear...
When it comes to subs and front sound stages you need to hear them in your room, there is no other way to figure out what will work for you.. Electronics, surrounds speakers, cables, stands, TV's, ect you can demo or see pictures, read reviews, check out at the store, and look at the specs and still make a good decision.. With subs and fronts you will need to hear them in your space, and compare them to a few options.... When I was figuring out my HT, I had to hang 10 different centers over my fireplace before I found the best one...
Obviously use reviews and recommendations as a starting point, and with a sub budget of $300, I have a feeling you will be between dual sub1200's, bicf12 or the polk 505 but still them are just starting points...
PS welcome to the forum, and whats with the name? lol...