I do not own Sonic Port, so I am basing this only on the product description of Sonic Port on this website. I think the answer is Yes - you can use it with Amplitube. The info I base this on appears on the Product Overview page:
... in the "Plug into iOS Creativity" section, where it says:
Sonic Port makes it easy to connect your guitar or bass to your iOS device and start making music. Simply plug your instrument into the 1/4" guitar input, launch GarageBand, the free Line 6 Mobile POD app, or another CoreAudio app, and start playing.
... so the question becomes: Is Amplitube a CoreAudio app? I believe it is, but you can ask/research that question in the Amplitube product web info/forum.
Update: I did a very quick and dirty search re: Amplitube and CoreAudio. There seems to be conflicting information. You should research this question yourself. The result will answer your question about Sonic Port and AMplitube.
Edited by silverhead, 26 June 2013 - 04:47 AM.