You might also consider the Hsu CCB-8. It has ported and sealed operating modes, and the sealed mode is probably better for the kind of boundary gain you will get from a crevice like a bookshelf. The thing about which side the port is mounted in a speaker doesn't really matter so much when you stuff them into a cubby The reason is boundary gain in low frequencies is going to get a boost no matter what, since the wavelengths of those frequencies are much longer than the bookshelf or speaker. What would behave better in those circumstances is a higher rolloff with a gentler slope like a sealed speaker. The horn-loaded tweeter of the CCB-8 would also be much less affected by diffraction than something that has a wide dispersion, i.e., a dome tweeter on a flat baffle.