Neka zapazanja, single vs 2 u paraleli... The 1W sensitivity won't change, but the 2.8V sensitivity will, simply because 2.8V will deliver more power to a lower impedance speaker. So in your case, you have an 8ohm woofer getting 1 watt. If you add another in parallel, the power going to the original is not affected, it's still receiving 1 watt, so it is still playing at 90dB. By adding the second speaker you've doubled the displacement which gives you a 20*log(2)=6dB rise, so the pair of woofers will be playing at 96dB collectively.