To add to Terry’s comments, using a tweeter opening as bass-reflex port is a really bad idea.
Port-tuning is higher math. The where, the size of the opening, the shape of the port, the length of the port all play a part in providing exactly the right back-pressure for the sub-speaker.
As for the differences between sub and full-range:
1) Subs will have/need crossover filter with very low x-over, typically 125Hz max.
2) Subs will play down to lower frequencies (as far down as mid 20’s on pro PA systems at events)
3) Subs are constructed differently as the enormous forces produced by the speakers cause quite a bit of reverberation and such in the enclosure.
4) Subs need loads of amp power as moving the amount of air at low frequencies takes lots of energy.
5) The driver in a sub is usually tuned to a very narrow frequency range (i.e. 30-150Hz), will have a different/larger magnet and voice-coil and often different cone and cone-rim attachment. All to be able to handle the heavy load. The low end driver in a full-range speaker will usually not go much lower than 50Hz (sometimes even higher than that) and it will cover a wider frequency range (in a 2-way speaker up to 1.5KHz for example), making it less suitable for the ultra-lows a sub is aiming at.
Hope that helps some.
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