The 3 rear vents has been designed for tuning the Bass-Reflex frequency of Murano Alto, and so on, its low frequency acoustical response.
It can work with two or three vents :
With three vents, the Bass-Reflex tune frequency is around 50 Hz and the acoustical level is almost constant between 50 and 100Hz. This is the factory setting.
With 2 vents, the bass reflex tune frequency is lower (around 40 Hz), but the acoustical level is non constant below 100Hz (in comparison with three vents there is+5dB at 35Hz, but -5dB at 50Hz).
Actually, the best setting depends of the room acoustic behavior and the speaker location. We provide a plug in order to close one of the three vents (We advise to close the upper one but you can close the one you prefer, the acoustical results will be very similar).
You can test both configuration with a music track with low frequency content (bass instrument), and choose your favorite solution.