Hi seven,

There is some compromise putting bookshelf speakers in a bookshelf, but it's not the rear porting that's the issue--a few inches behind the rear port is sufficient to let the speaker vent. Rather, it's the adjacent boundaries of the shelving that you don't want interfering with the output of the drivers, which is why we at Axiom suggest you have the front baffle of the speaker protruding from the bookshelf by an inch if you can manage it (a couple of centimeters in metric). If you do that, you'll keep compromises in sound quality to a minimum.

Alan Lofft,
Axiom Resident Expert (Retired)