It is 2" or 2 inches
There owners manual recommends ideal placement at 27" though from the wall. I only had 8-9", that is why I went with a sealed design vs a ported design. I did call and speak to an HSU engineer and he did assure me that these speakers would work in my space as long as I had at least 2" of clearance. But I did not want to create any limitation since I do not have the fake wall built yet and the acoustically transparent screen in place. So I would not be able to send them back and get my money back within their guarantee window.
And in your case (since you will be in Japan) you might want to play it safe as well and go with a sealed design.
You are going through the exact same thing I did
That is why I went with the SVS speakers. At the end of the day they met all my room requirements and get rave reviews. If you want to read more about SVS this site was very helpful as well:
http://www.hometheatershack.com/forums/svsound/