From the link:
*This* is the difference between solid engineering and these 'boutique' (a.k.a. electronics beginner with delusions of grandeur) devices.
You could be listening to the output of an iPad or other competent device, but instead you spend real money on a hobbyist amateur's box that uses two PP3 batteries that only last for a couple of hours and then starts squealing, oscillating and 'popping' for entirely predictable reasons.
It's the same as the guitar fuzz box I made for someone when I was 15, using two PP3 batteries (and double pole power switch) because I didn't know enough electronics to build a circuit that ran on a single battery.
I love the way the instruction sheet says it will only start damaging your headphones after "an extended period" as though they have characterised the effect with all headphones, at all temperatures, with all possible batteries, etc. and it is their highly professional assessment that no harm will be done until you are several pops in. Just don't fall asleep while listening, or try to play a third or fourth album. No pressure.