The problem with your frequency response on your music was likely your box. Be sure to design your enclosure around your musical preferences.
Are you sure the sub isn't damaged if its gotten so much worse over time? or are you just ready for something new?
If you're spending up to 700, minus 200 for a box that leaves 500 for subs or 250 per sub and in that price range, I would have to recommend the SSA GCON for a sub that gets loud and sounds good doing it. They will take everything your amp has to give and more if you ever upgrade your power; but they don't "need" it to perform well.
Or if you don't quite want to spend up to the top of your budget, look into the new Alpine Type Rs. Very impressive and once again will take every bit of your amp plus more.
Those would be my recommendations if you really just want to get new subs and both fit your budget.