I've dealt with this stuff before, and know a lot of eBay sellers that have also. And you made the right decision IMO. This was the smart play. And your good nature served you well. Had the seller chosen to go to PayPal for a refund. They would have tried to get both of you to pay return shipping, and see who blinked first. And then after a short period of like a month or so they would have refunded the buyer the entire amount.
Now if you both chose to be stubborn. He would have gotten to keep the speakers, and the refund. But I'm assuming you would have paid the return shipping. So you would have most likely paid the shipping both ways, had to buy the new grills anyway, and got stuck dealing with having to sell the damaged speakers again. This would have surely cost you more then the $200. A lot more!
Yes you got screwed out of $200, but it could have been worse. I would chalk it up to a $200 lesson that might save me money in the future. Unfortunately this is the reality of selling online, at least when using PayPal. And now you know why you see some sellers on forums that refuse to take PayPal. Unfortunately you have no choice on certain platforms, like eBay. But that's a whole other story there.