Cabin gain is the bump in output at certain frequencies caues by the car itself (due to the shape of the vehicle, and the internal air presure and the reflective surfaces).

 

I have to admit I have never bought into the idea of putting my mids at 120 and subs at 80 down -- the sound always seems a little "thin" to me.

I do things a little differently.... I run my fronts full rangle into the component crossovers and the sub 80hz down.

If I was doing an purely electronic xover I would go 50hz or 60hz up on the mids and 2hz on the tweeters to start -- obviously, I would listen to the car and make adjustments if necessary.

Just for the record...I would not use these settings for someone who listens to rap or metal or freaking loud -- just old guys like me who like a lot of Jazz and Acoustic music.