I would have taken a different approach after the unsuccessful attempts at the store.. however.. I am not a lawyer or anything so take this info for what it's worth.
Right at first... I would have talked the shop owner, not the salesguy, that because their shop caused damage to your property especially when they were not authorized to do so (wiring up your sub), that they (the shop) are liable for any and all consequential damages resulting from it within your states laws. I would ask that your property be replaced immediately. Do not make this come across as a threat but merely to disclose to them your intended direction of action and your expectation of their resolution. Thank the shop owner for his understanding and that someone will be in contact with them shortly.
Now go home and type up a very professional letter (google legal letters examples if you need help) re-itterating the same things you already spoke with the owner about. But make it very simple and to the point.
1. You were asked to leave you Brand New product by their staff member merely for measuring purposes. Be specific with names and dates.
2. Staff personnel without your specific authorization used and damaged your Brand New product.
3. You fully expect the Shop to cover the full costs of replacement of your damaged property due to the shop's negligence.
4. Advise that an invoice will be sent shortly for the damages.
5. Also state that any further contact from the shop, it's staff, and/or owners is to be made by mail correspondence only. (It is very important that you state this)
Mail the letter CERTIFIED MAIL. Also get the document notarized by you local bank official or Notary Public. This involves them stamping and their signature on the document.
Next I would send them an invoice demanding payment from them by certain date for the cost of replacement of your damaged property. This would be considered a bill that is due payment. Have this invoice notarized as well. This makes it look very official and clearly says that you mean business. Also have the invoice mailed to the Retailer listing the owner's name and shop name as the person addressed to. Also make sure to mail it "CERTIFIED MAIL" with return signature label card. What this does is forces the mail carrier to get a direct signature from the shop owner and proves that the invoice is received by them. The most important thing this does is it clearly says that you are serious and that you are building a paper trail. Everybody in any kind of professional business knows that when there are signatures involved and a clear attempt to make and keep records of a paper trail, they know that some LEGAL PROCESSING is in the works. Usually this is enough to get the shop to wise up, as no shop wants a lawsuit that they will have to defend no matter how big or small. It's time consuming. It's costly and it's bad for business. Doing these steps will let them know you mean BUSINESS and your not just a little kid they can push around. More than likely, this will be as far as you need to take it, and the shop will agree to make things right as to them, it is cheaper than the alternative.
If they choose not to, your next step would be to go to the county court house (where the shop is located in) and file the beginning of a small claims suit against the shop. Again, this is just more LEGAL paperwork and should be more than enough to help your cause without ever having to go to court for this amount of damages. I doubt that you will even have to go this far.
This will be far more effective than you (just some kid to them) calling and yelling and being all upset at them.