Volume is usually controlled between the pre-amplifier and your power amplifier. So, in your case, it should either be the iPad or a potentiometer you put between the iPad and the tda2005. With all of the capacitors on the output of the recommended tda2005 circuit, you will be creating high pass or low pass filters if you try to control the volume on a feedback loop of the power amplifier.
I would definitely not use a 2005 as a volume control. would jerry rig some kind of pot inline from the output of the IPAD to the input of the amp. The output of the IPOD can probably easily drive a pot with impedance in the range of 100 - 1k Ohms. make sure it is an audio taper pot.