It seems like what you describe would work... so your description is puzzling. However, even if this would work, it would hardly be worth the sonic degradation on your system. The quality of the sub-crossover is probably low, and it would rob a lot from the signal feeding your power amp.. and affect the quality of the sound from your main speaker.
I had this dilema once with NHT super zeros and a sub, where I opted for making simple passive high pass filters in line between the pre and the power amp, resulting in much better sound than using the Sub crossover. I still have these sitting arround, and would be happy to pass them on to you for free. However, the Impeadance of your poweramp would determine if you could use them or not. The cut of is about 127Hz when used with 38Khoms impeadance (ROTEL), but would change depending on the impeadance of your amp.
I had this dilema once with NHT super zeros and a sub, where I opted for making simple passive high pass filters in line between the pre and the power amp, resulting in much better sound than using the Sub crossover. I still have these sitting arround, and would be happy to pass them on to you for free. However, the Impeadance of your poweramp would determine if you could use them or not. The cut of is about 127Hz when used with 38Khoms impeadance (ROTEL), but would change depending on the impeadance of your amp.