An easier solution in your case would be a REL sub, which does not divide the signal going to the mains, but instead simply adds in low bass. It hooks up to the speaker output terminals on your amp. It works extremely well. The 2Wq is a good sub but the RELs are very good as well.
how to connect Vandersteen 2Wq to integrated amp
I have the Musical Fidelity A3 Integrated and Dynaudio Contour 1.3SE. I would like to add to the system the Vandersteen 2WQ Sub, by profiting of the preamplifier autput of my MF A3. However, the crossover of 2WQ is divided into separate low pass and high pass sections. The high pass section should be inserted into the signal path between pre- and the main power section of the Amplifier. Could you advise me if it is possible and how to make this modification.