Headphone only mixing is never a good plan. Personally I only use my headphones to listen for specific problem area’s in one track or when comparing two tracks.
Getting a sub with your monitors might not be the solution either, as they often don’t go any lower than 35 or so Hz (some even down to only 40Hz). While 25Hz more than your headphones, still lacking that last bit.
By the same token, car stereo’s and cars in general are not the best listening area’s. Often car stereo’s are over-subbed and EQ-ed. It seems that the more the metal on your doors goes in and out in tune with the music the more macho your car is. Never mind 80dB safe listening levels.
What many studio’s do is have multiple monitors, like a small mono one (to see what stuff sounds like on a mono device). You could even hook up a car speaker to your board if you like.
Something simple like a pair of KRK 6s could be an option (that costs less than a single sub), also giving you a bit more power and frequency down to 38Hz. I’d stay away from the 5s as they will lack in bass a bit.
Clearly the 15Hz of your headphones is a nice sales pitch, but bar a few new-born babies, us humans don’t hear much below 20Hz anyway :-).
In general (while I think the A2+ are nice) I believe you are slightly underpowered with your current speakers and you should definitely move up to something able to deliver something in the 35-38Hz range. You just lose too much information in the 35-60Hz spectrum to be making good mixes.
If your speakers do play a frequency but it’s not a flat curve, you can compensate for that with experience, but if the speakers don’t play the frequency at all, there is nothing to compensate.
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