Above the resonant frequency of a bass reflex vent, a loudspeaker behaves identically to a sealed box, in that the vent doesn't affect anything. You can see this easily if you play a, say, 1kHz tone on a vented loudspeaker and block the vent. There will be no change to the output of the 'speaker.
At frequencies below the vent resonance, the bass driver is unloaded, so there's no restoring force, which is why bass reflex loudspeaker cones flap about on Vinyl playback if the phono stage doesn't have good LF filtering.
A 15" unit used up to 950Hz I suggest is bad idea as it will be beaming significantly by then, I wouldn't let a 15" unit go above 200Hz or so, and cross over to a 4 or 5" by then. As a 'rule of thumb' I would have each driver covering no more than a decade, so LF 20-200, MF 200-2kHz, HF 2kHz up, although probably better as a 4-way or even 5 way if one genuinely wants to get down to 20-20kHz flat with any roll-off outside that.
Tannoy 15" Dual Concentrics have their crossover frequency at 1kHz, which I've always felt was just plain wrong.
S.