Excellent!
Check out the DIY sub-forum (which is more or less a DIY-Sub forum since subs are such a high value diy project)! Give a budget, any size restrictions, whether you are going to use for HT (do you need a LFE subwoofer or just a good solid woofer) to get started and the guys will set you up with a design to fit your requirements!
A good example of a music only subwoofer is the 8" subs
@MrBoat just finished which would do fine as long as your room is not too large.
If I was in your shoes, I would target my budget for dual subs. The DIY savings should buy the second sub, and it is pretty efficient to make 2 subs!