🚀 go-pugleaf

A line-level output is high impedance, and just isn't designed to put out lots
of power.  You shouldn't be drawing more than a milliwatt.  That's not to say
you can't, but if you push it you'll get some distortion.  A transformer may
shift the impedance, but it won't change the power situation.

What to do?  Many op-amps drive headphones just fine.  Not just any random
one, you want `high output' so check the specs for output power.  You probably
want some voltage scaling, so you need two resistors too.  If you want to get
fancy, one can be a pot, and you get a volume control.  Total parts cost could
be under a buck, assuming you hook into the TV's power supply.