Even larger sized piezos may only go down to 2 or 3 kHz with authority. This frequency is too high for a 15" woofer to run up to effectively. Go for the compression driver.

As far as the ports go, well how many 'ways' a system has has nothing to do with ports. A 2-way system simply means that the frequency band is split into two portions, each diverted to different speakers. So a 2-way system would have a woofer and a tweeter. A three way system would have a woofer, midrange and tweeter. Etc.

Adam