^^^ +1
If your room isn't well-treated, a sub can just complicate things. Many of them come equipped with a bypass footswitch jack so that they can be easily taken out of circuit while mixing. It's a good feature to look for when shopping for a subwoofer.
But before you start shopping, do some basic testing to see if you even need one. Start by measuring your speakers' frequency response. All you need is a clean sine wave audio source and a microphone to record the speakers' output. What you'll observe is that output will start to drop off dramatically at some frequency. Find that frequency and compare it to where the musical meat of the low end lives, around 60-80 Hz. If your speakers drop out at, say, 50 Hz, then I'd say you probably
don't need a subwoofer. Don't worry that they don't go down to 30 or 40 Hz,
that is not going to ruin your mixes.
So what to make of it if your speakers seem adequate but you still have translation issues? In that case it's not about what you can or cannot hear, but rather about the
evenness of what you hear. Not just in your studio space, but also in the other environments you're comparing against. It's almost never a speaker issue, but rather an acoustical one.
Cars have the worst acoustics outside of a drain pipe, so be careful about conclusions based on car tests.
I transferred my room test sine waves to a CD so that I could take them out to the car. I drove my car around to the back of my garage studio so that a mic cable would reach it, then stuck a boom through the driver-side window so the microphone was positioned at ear level. I then played back the CD and recorded the output.
I expected it to be awful, but wasn't prepared for just how bad it really was: 30 dB peaks and valleys, and a huge bump at 120 Hz. And it wasn't just low frequencies that were seriously messed up. Comb filtering from tweeters aimed at the windshield sent the frequency response all over the place. Mastering a mix so it sounds OK in the car is a worthy achievement, but ultimately means nothing because it will likely still sound bad in another car. More relevant to this topic, it would make no difference at all whether or not I used a subwoofer for mixing and mastering.