I have done this before where a headphone jack was used to drive other audio equipment. I haven't burned anything up yet, but I also set the volume to 50% output on the device that has the headphone jack so that its not overdriving the amplifiers input of the daisy chained device. Then set the volume to maximum on the device at the end of the chain and test for max volume and quality. If its loud enough and good quality then leave it there at 50%, however if it sounds too low you can raise the level until you get the maximum volume you want however as your raising the volume your also risking overdriving the input to the amplifier of the next device. Additionally if you raise the output too high from the headphone jacked device that is driving the daisy chain amplifier your audio quality will turn to garbage because your amplitude of your output starts to get clipped by the maximum input amplitude so if you think of a wave form that is say 5 volts such as if you had an oscilloscope and looked at the wave form and saw it swung 0 to 5 volts and the maximum input voltage to the amplifier was 5 volts and your now pushing 6 volts into the input, the signal from 5 to 6 volts in amplitude will be clipped and so the wave form will get flats at the top and bottom of the wave. And this will make it sound like garbage. You want to find a level of input that never overdrives and distorts the output. If you hear distortion of the audio then you need to lower the output from headphone jack to the input of the next devices amplifier.
Lastly... There comes some risk with this setup that you could burn up the input of the amplifier by overdriving the input. But from my experience being careful not to overdrive the input I have done this for years without any troubles. If your willing to assume the risk of damaging your equipment and careful it can be done.
