a newbie filter question

Ok, I have just finished an active sub which I'm going to run from one of the line outs of a TEAC micro system. I now need a low-pass filter for the line signal before it is amplified and would like to know..

should I use an active of passive filter?

As I am using a filter for just the low frequency is there likely to be audible phase difference between my sub and my other speakers?

any help or guidance would be greatly appreciated.
Just out of curiosity, do you already have the amplifier that you are using for the project? If not, you can always buy a plate amplifier, such as I did for my current Sonotube project. The amp comes with separate phase and crossover knobs, so that you can adjust it to any setting you desire. Doing so can allow you to adjust your crossover point without knowing exactly where your mains roll off in the low frequencies, thus providing seemless integration. If not crossed over at the proper frequency, there WILL be an audible difference. If you cannot take the plate amp route, then I would suggest going with the active filter.