A:Answer"Best?" That's, ahem. . . a loaded question. In most simple terms; ANY self-powered (active) sub can be used. It's just a matter of hookup method. This unit has RCA-type output for the sub. Lookup (google) articles on choosing a subwoofer. Be prepared for numerous and varied opinions, however.
A:AnswerMost TV's have one HDMI input that is and ARC connection, that goes to the same input on the back of the receiver. That runs your TV through the receiver at all times.
A:AnswerHi, geoff! For more information about the manufacturer's warranty, please refer to this link:
https://docs.sony.com/release//Warranty_459910711.pdf
~Ralph
A:AnswerSounds like the speakers are wired in such a way that is lowering Ohm below supported levels. I'd attach a single pair of free-standing 8 Ohm speakers to check for sure.