a007udio said:
I would like to use Linkwitz transform but i cant becouse i would like to use only one integratet amplifier. So no electronic eq and stuff like that....
Seriously consider giving the sub its own amp. If it's not a dual voice coil driver, you could only use it on one channel anyway. The inductors required to get a suitably low crossover will be huge, expensive and have relatively high DC resistance, causing some shift in your alignment.
Plate amps are fairly inexpensive from a variety of sources, or you could make a 200W bridged/parallel chip amp (National's LM3886 app note provides a suitable circuit that should be sufficient for subwoofer duty.) If you made you own plate amp on a couple of flat section heat sinks like Steve at apexjr.com has it would might even cost less than good quality inductors.
Count me as a member of the high quality PA driver subwoofer fan club. I run a pair of JBL 2245Hs (18") in ~250L ported cabs. They sound great and can take anything I can throw at them, but with the EQ required each needs 500 Watts minimum for headroom. Having two subs in non symetrical locations (mono signal) really helps smooth the response in my room.