Your next email to him should be...
Thanks for the offer, but I would much rather just return the amp to you, in the exact condition I received it, than have it repaired. I spoke with paypal and my credit card company, and I was advised to return the unit to you and get a refund. They did offer to perform a "charge back" but I assured them we can come to an amicable conclusion without resorting to that... Please let me know what address to return it to, and I will send you and paypal the tracking info as soon as I get it. Thanks for your time, and I am sorry this didn't work out better...
PS if you give me any trouble, I am going to contact my credit provider and get a refund, then send you the amplifier full of dog excrement.
You can leave the PS out if you like, but I think it adds a certain amount of panache...
In all seriousness get ontop of this, dont let it go too long, even when he agrees to take it back, contact paypal and have it returned through there system. This guy new the amp popped and made noise, thats why he was selling it..