After I upgraded from Amplitube 5.1 to 5.2, some of my presets started having intermittent hiss. It would happen every 5 seconds for a couple of seconds. I was able to track it down to a speaker in a cabinet that was not authorized, and it would appear to be the 3rd and 4th non-existent speakers in the cabinet. In the preset file, which is xml format, there is a model for the speaker listed for the 3rd and 4th speaker.
I found this by making my own version of the preset that was at issue, then did a file comparison of the two.
The solution is to select one of your 4x12 cabinets, then switch back to the offending 2x12 cabinet, which, now has the correct 1st and 2nd speakers (its original values) and values for the 3rd and 4th speaker that, although not actually used in the preset, are now listed in the preset with values for speakers that you actually own.
THIS IS A BUG. The software engineers should only load values from the xml file for which there are actually elements being used in the presets. That way, for a 2x12 cab, it wouldn't care what was listed as the 3rd and 4th speaker in the preset, since it doesn't actually use those speakers.