I haven't had any issues with my 8i6, running very low latency. I have the 2i4 as well but have not used it yet with Amplitube, it has worked very low latency on all other VST's using ASIO drivers. My biggest issue with the 8i6 unit was at first I forgot to set the Line In as an instrument so I wasn't running it as Hi Z.
Don't forget to watch the input and out put levels with in Amplitube as well. You don't want to clip in the red on the way in or out of the Amplitube, the settings are along the bottom. On some of the amp models I actually have to adjust the input up a little within Amplitube to make sure my signal is shot enough. It all depends on how hot your pick ups are.
Also, there are hi, medium, and low resolution settings at the top in A3, to help with latency. You can set them low for tracking and reset them in Hi for the final mix or bounce.