I installed my HU yesterday, a Blaupunkt Santa Fe, and the guys who helped me out worked on this sort of stuff for about 12 years. He did car audio, and worked on concert lines which are very similar, so I trusted him. He installed it within one hour, and everything works perfectly. While installing it he asked if I had subs and an amp and I told him yes, but I hadnt got the wires to install it. He told me that when I was to put them in I needed an electronic crossover to make sure the speakers inside get OVER 120hz and the subs get the 120hz and UNDER. I just wanted to put that he doesnt work for any company, hes my freinds dad, meaning i would expect some one who works at best buy to try and sell me these sort of things even if it wasnt really needed. SO i am asking you al, is it necessary?