The headphone out will work with any amplifier whether is has RCA or 3.5mm input. You just need the correct cable. The headphone out can be higher than the input of an amp will accept without causing overload distortion so you would start with the TV volume all the way down and the amp volume at about 12 o'clock Then turn up the TV volume until is is pretty much as loud as you would ever use it. If the TV volume is to high you will hear distortion at any volume you play the amp.
You might consider getting a surround sound receiver (new or used one) that will have optical inputs. It would sound better and allow you to expand the system when you could. An older non-HDMI receiver would not be expensive. Check Craigslist, garage sales, and...