If you're not after the very high SPL that these guys think you are (

), the
Adire DPL12 is my favorite sealed box (what most consider when they think "music sub") woofer. Stick it in a nice ~75L box with only about 150-200w (I run mine with a PE 150w plate amp) behind it and you've got quality as good as it gets short of a big dipole setup. Check out
this thread for Jon's typically detailed anaysis along these lines.
As you see from the link to the driver, Kyle has them on sale for $125 shipping included. Given the modest power requirements, it's quite an economical setup also. If you think you want more volume, multiples are an option, as also discussed in the thread above.