I connected an active sub to my amp in series with my bookshelf speakers (they share the same terminals). I then adjusted the subwoofer crossover to the lowest frequency that the bookshelfs can handle, I think the idea there is so that the subwoofer will take over those frequencies.
For the impendance, CMIIW, but I think wiring the speakers in series actually increases the impendance and lowers the load on your amplifier. If you wire full range speakers in series, this will result in lower power to the speakers. But since you're wiring a sub which will only take over in low frequencies, I think any power loss is negligible. I didn't notice too much with mine in case you were wondering :-)
from what i gather from this and other posts, i have two options
option 1
speaker terminal (amp) ------> powered sub
------> (bookshelf) speakers
option 2
speaker terminal (amp) ------> (bookshelf) speakers --------> powered sub
i think both options would mean i would be wiring in PARALLEL (right?), which would mean i would be halving the impedance (ie. if sub is 8 ohm and speakers are 8 ohms, load for both would be 4 ohms), right? don't have the specs of my bookshelves (Anthony Audio Proac D1 clone) but i'll try to measure them later with a multimeter.