Low Pass filter selection

I have a mofo 15, how many hertz should the low pass filter be? I can not find any spec's on the frequency response of a mofo.  I was thinking about getting a 100 hz or 150 hz, does that sound alright?

Thanks,

Arturo

People will normally adjust the low pass crossover "to taste", which is to say, adjust it according to the quality of the overall system sound.  The high pass for the main speakers is adjusted along with the sub's lowpass, and when you are done tuning the sound by ear you will likely find that the two filters are at just about the same frequency at about 80 Hz.  But the crossover point could be up to 100 Hz, as well, and that would depend on the quality of the front speaker setup.

I like to use sine wave test tones for setting crossovers, which allows more accuracy and quicker tuning.  If you set the sub crossover too high, at least two symptoms will appear:  there will be too much of the higher frequencies being played by the sub which begin to sound "directional".  That is to say, you would be able to pinpoint the sub location.  Subsonic frequencies are omnidirectional and should feel as if they arrive at your ears from all locations.  The higher freqs are provided by the mid drivers in the car's front soundstage and will divide the left / right stereo sound properly to your listening location.  A well-tuned crossover at 80 Hz will allow omnidirectional subsonic sound while providing emphatic punch to the stereo soundstage.  This goes to emphasize the importance of setting up a good front system along with the addition of a sub.

The second of the symptoms would be an over-emphasis in the frequency band of lower midbass reproduction.  The sub lowpass set too high will add sound that the mid drivers are already providing, which creates a "hump" in the response.  If you tuned by ear, you would find the overly loud band of music objectionable and would adjust the hump out of the response curve.

So you've got the low pass covered for the sub.  That eliminates one needless purchase.  And your head unit should already high pass to the fronts, come to think about it.  Shouldn't it?  Do you need any additional filters at all?  It doesn't look that way, as I think about it.  You should set this up, use the lowpass on the mono amp for the sub, and see how things turn out.  Bass blockers are cheap electrolytic caps that provide a crappy 6 db crossover.  You can buy one for about $3 ( http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=027-370).

You would know if you have to go that direction if the front speakers complain with low notes.