When I had my i-MiEV leased for two years, I put in a small Sony amp under the rear seat (where the useless tire inflation kit was located) and a smallish 10" single sub in the rear hatch. The Sony accepted speaker-level inputs so I didn't have to change the stock head unit.
I don't see a good place under the front dash (and NOTHING under the front seats) to put a small sub. The sub enclosure I used was small enough to be portable. If I needed the extra storage room in the car, I would just take out the sub. You may have to do the same thing to get decent low-end response.
That being said, I have NOT made the same modifications to my LEAF yet. Since I'm working from home now, my need to improve the stock radio is less since I don't spend much time in the car any more. Given the head unit in my "S" model is the same manufacturer as was in my i-MiEV, I expect the same good results if I do make that modification.
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