It's easy as an app from itunes ...

IPhone and iPad. Amplitube and Ampkit

It is stable, some noise on iPhone, less on Ipad

I use it for today ... for 3 hours! I found it mediocre at the beginning: no power, feedback galore, I was ready to go to the retailer, then ... I boosted the volume of my two devices (jailbreakées. ..) and I was able to remove these feedback. For the price it does, it's convenient, it sounds pretty and it will be especially convenient to have a little sound to my classes without taking anything but my Ipad and my iphone.