I have built my own speaker boxes plenty of times, and I have also replaced worn out speakers, just like the situation you are in. It is very easy to do, if you want to do it yourself......then it would just be the cost of new speakers, which have a high range in prices. I would just get something in the mid-range price. You could research EBAY or your local audio store.
Click here to see how easy it is to replace a speaker.
If you are not comfortable replacing them, then you would have to bring them to an audio store. Some repairers may even come to you.....it would be worth researching, since you say they are heavy. I do not know what the labor cost would be, because I have only repaired my own, but it shouldn't be that much since I can replace one in about 5 minutes.
If you need further help or clarification on this question, please reply to this post.
Best of Luck, Brian
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