HU-850 to HU-650 swap.
 

Hi. I have changed my broken HU-850 for a HU-650. I have now got very bad audio through the speakers. It sounds as if the speakers have blown. If this a fault caused by using the undeseat amp?

The HU-650 wasn't (as far as I'm aware) meant to be used with the underseat amp so you probably need to try swapping out the connection for that so the speakers are powered directly from the HU.