I have to say you are right to a point, but speaker choice is down to a few things,
1) Space, like you said you need to have enough air to move to make a good sound
2) Music Genre, different speakers and different enclosures will all give different sounds so what will work for one style won't necessarily work for another, for example I now have a subwoofer in my car, and it's good but I'm not completely happy with it, it's a fusion 10" driver and ported box with the amp all in one and all though it give great low end it's too boomy because of the ported enclosure, I listen mainly to heavy metal (Metallica, Disturbed, Killswitch Engage, Papa roach etc) so really what I need is a seal enclosure to make the bass sound tighter and more responsive to go with the kick of a bass drum and the fast guitar riffs, (which a 10" sub and a sealed enclosure will give) on the flip side if hip hop is more your thing, which tends to have slower beats and lower, longer bass tones then you want a bigger subwoofer and a ported enclosure to produce those big boomy long tones,
3) Personal choice,
What I just wrote is a guide line which works for most people but of course there will be people who won't like certain tones and amplitudes to intrude when listening to some music, for example I also like muse, specifically there song "Hysteria" which has a really nice bass riff and my set up that I have at the moment completely ruins the sound of it, when he changes the tone of his instrument when the whole band starts to play together after the intro when Matt Belemy plays his seering guitar riff, it's just far too thick and no matter what I do I can't change it,
So.... You need to go to shops and listen to different speakers and choose the one that you like the best and always buy the best you can afford and one that you are happy with,
that's probably all the advise I can give you into that
and kenwood do shallow subs so check them out