Personally I wouldn't turn it all the way down, especially the way you say you play it. 1/ you'll end up burning your sub up, 2/ forcing the sub to play what's not contained in your audio will get you nowhere. Hell, I wouldn't even have it at 25 jmo.
things to consider.
1/ when others play music and they say it hits great, you don't know...sometimes they don't know...how their overall system is actually tuned---there's your difference
2/ one song hits harder than the other a lot of times simply has to do with the two songs being recorded and mastered differently---there's your difference
3/ (assuming you're using mp3...or mp3 vs better format...or your mp3 vs friends cd) lower bitrate cuts off freq, especially bottom end. Even if the track your playing claims a higher bitrate, it might actually be lower and up converted. Also, not all encoders are made the same, so two encoders doing 320kbps, one of them can be crap---there's your difference.
4/ your car itself can be absorbing certain freq---there's your difference.
5/ you could have some freq cancelation going on in your car and simply need to play with placement---there's your difference.
You will never have a music collection that all hits the same everywhere unless you remaster each track to how you want. I had to do that with about 30 of my tracks and all is good. Not everything can be fixed by the box, fixing that for one is simply going to throw off another track. Sometimes you should look at the source and/or environment. What you should do is 1/ tune your system to a middle ground for all your tracks and mess with your base knob every once in a while...never if you tune it right 2/ look at your source material/environment.
Note to source material, if you're bumping mix CDs, or copy's of them, CDs aren't what they used to be. A lot of them are simply mp3 burnt onto CDs.
My couple of cents, im sleepy so some of this might be incoherent, gl.