The problem with double amping is that even with a volume control you'll have to feel your way around finding the proper setting on it first, then the second problem is that aside from just the gain you'll have to deal with the headphone output having impedance designed for earphones while the input of the amp is designed for a line out signal. Then there's the problem of how clean the phone's line out is.
You can avoid double amping and also treat the signal a lot better (more digital, less travel of analog signal) if your Android has OTG USB Audio support. Look up the thread in the Portable Source forum here to check for compatibility, and also where to get a USB cable already wired for OTG so you won't use the bulky adapter. This way the digital signal goes from the phone then into the DAC-Amp where it is decoded an amplified. Amplified signal doesn't even leave the circuit board until it gets to the headphone cable. Check out the Fiio E18 - I think it has the widest support for Android devices. I think it even comes with an OTG cable.