Unfortunately the best replacement for your Bose will come from Bose. Yes, the woofers are cheaply made but unless things have changed, they are made specifically for Bose so a generic replacement from another manufacturer will give you an even more unbalanced sound.

If these are 2008 models they are still under warranty for the original owner. If you arent the original owner try calling them and maybe they'll cover the replacement anyway.
Good Luck,
crazychile