One comment to the OP. Just something to keep in mind. When you make your box smaller your resonant frequency will become higher. Of course, same goes by changing to a smaller speaker. However, some of this can be overcome by better subs that can push more air and by possibly using a ported box. Then opens up the possibility of blowing your subs because now your subs can have a greater excursion.
Some of this may seem obvious to some of you, but I think sometimes it not jsut about the power,etc. Like Regina points out you don't always need the top of the line sub. A lower of middle of the line may do better and you may get more sound of of them. Since you are talking to an audio shop they should know this, jsut try to sense if they are jsut trying to sell you something. I know you started this thread just changing to a used set of subs and now you are looking at new subs, amps, box, etc. Good luck, let us know if we can help you more.