There are many audible subsonic harmonics (and partials) in most program material.

Knee jerkedly cutting off all sound below 60 cycles will rob your music of some of
it's fullness (even when what you are hearing is not distinct).

Also, be careful in judging your music on car stereos.

Most car stereos have very exaggerated subsonice (if they, indeed, have the capacity to reproduce those frequencies at all).

Here's what I have done to begin to 'learn' how my own monitors (NHT Pros) color the sound of a mix.

I listen to a mix on a cheap boom box
I listen on an expensive boom box
I listen on a cheap car stereo
I listen on an expensive car stereo
I listen on a cheap home stereo
I listen on an expensive home stereo.

Finally, I cross reference my mix by listening to it on a pair of Sony Professional Headphones (which are an industry standard
and mercifully, only cost $100)

This last test will accurately let you know what your subsonics sound like (as they are flat from 20 Hz to 20 K Hz, below and above
a typical human being's ability to hear frequencies in music).

After using this method many times, I don't use it at all any more because I have gotten to 'know' how a mix will sound averaged
over all those many systems.

Now, I just mix till I like it (without a subwoofer) and occasionally cross reference to the Sony headphones to make sure that
I'm not killing the mix with two much subsonic saturation.

Be careful of one thing, though...............DON'T MIX SOLELY ON HEADPHONES!!!!!
As good as they sound, they do not let you hear cross ear information that really informs a stereo image when listening on speakers.

All your reverbs will be incorrect if you only use headphones because of this lack of cross ear referencing.

And one last little trick if you have mudiness between two bass sounds (usually kick and bass synth or guitar)

Use a 31 graphic equalizer styled plugin (or an analogue one) and put each channel of bass into one side of the equalizer.
Now, take the lowest frequency band of the first one (should be 30 cycles) and lower it 3 DB. Commensorately, take the 30 Hz band of the other instrument and raise it by 3 DB. Next move to the next band (60 Hz) and do the opposite............boost it by 3 Db and
cut 3 Db on the opposite instrument commensorately. proceed all the way up the spectrum on the graphic in a
zipper like fashion.

This is an old analogue trick called 'Zipper EQ' that I don't hear used much any more.

What kicks ass about it, is that if you solo each mono bass track they will sound a little weird but because Bass frequencies are
omni directional (mono in essence) when you put them together, they will sound really good together but distinctly different from
each other. The human brain will fill in the frequencies missing in a psycho acoustic way.

And one last piece of subsonic mixing advice and I'll shut up:

Put on your headphones and go to the 60 Hz band on the whole mix and take it completely out.
Now, slowly bring it back up again. As soon as you can feel the band you can stop.
Experiment with taking it up and down. Just feeling it before you actually hear it will put enough
of these frequencies in the mix to make the whole thing sound more naturalistic.

Now do the same thing with the 30 Hz frequency bands.
and raise it by 3 DB.