Here's what I suggest you do:
Stay at arms length. The shipping company will ONLY work with the shipping party (ie not you). Use this contract to your advantage. Inform the seller of the damage. Send pictures of the damage, and describe it in detail. I would NOT assign blame. Simply state the facts and ask him/her to inform you of what to do to help them remedy the problem. If the amp appears to be beyond repair, say so. If you would like to return the amp and be done with the transaction, say so and explain your reasoning is that the amp is no longer of interest to you in it's condition. Be patient, objective. and persistent.
If the seller says it's your problem then you're into irrational territory, and you'll need to argue. If you have any way to get your money back (ie Paypal buyer protection, etc) you'll need to do so.
I've been in similar situations more than once, and the strategy in my first pragraph has worked because the sellers have operated in good faith. But I NEVER told them I thought their packaging was crap, even thought it was.