Agreed. Contact Ebay to tell them the speakers have arrived and ask what you should do next. Do not return them without further advice from Ebay. He could scam you, saying they arrived back damaged or not at all. Some people have even suggested that the returned item was a damaged substitute. Be careful. This guy sounds dodgy. Message Ebay and follow their advice.I would do everything through ebay and not communicate with the guy at all..
If you return the speakers to him privately, he might ask you to 'return' them again..
I wouldn't worry about the defamation and solicitor threats. The guy is talking non-sense and just trying to intimidate you.