Kai: I did not offer a solution I asked a question. To me this sounds like a transformer matched PA system . I wanted to determine if he had a defective replacement speaker. Putting two in series will affect the hi fi quality but it will also make it obvious if one of the speakers is damaged.
The other issue with speakers is the "efficiency" issue different speakers driven with the same input can produce significantly different outputs due to construction differences such as magnet quality ans coil to magnet clearance.
Terry