First, you'd better define the reason for this null at least with REW RoomSim and see if adding the sub will help at all.
In fact, if you have mic and something like DSP for manual correction, most probably you'll be able to make 2.1 with steep crossover maybe up to 100 Hz (if your sub is located not in front of you).
If you will use shallow slope, you'll have too much midbass sound from subwoofer and it will localize itself.
Or you'll need to get subwoofer with possibility to use high order crossover.
Without more details I can't recommend anything more specific.
In my car I play under seats to 170 and cannot localise the midbass drivers, and they pan around the stage with the midrange very nicely, it’s all about phase integration, if you hear then at 100hz to 150hz it’s certainly phase and level integration and not the crossover point
Also if you have phase issues (from standing waves and reflections) at the crossover point ie 80hz in this case then move the crossover further away from it, the phase is not great where the null is, and crossovers depend on phase being good through the crossover region
Smaart (any version, I have V7, di2 and V9 now) is a massive help for speeding up setup times for dsp controlled cars and houses both