To offer my 2 cents: 2-3dB can be quite significant as you go down in frequency. As you'll note in this article:
The Decibel (dB) Scale & Audio Rules 101 — Reviews and News from Audioholics
As you go down in frequency, it takes less additional output to achieve a perceived doubling of loudness at 20Hz than it does at 1kHz (though as you go up in volume, this effect does lessen looking at the equal loudness chart). Moreover, since you feel 20Hz, it's worth noting that a 3dB rise equates to a 40% increase in pressure, and a 2dB increase equates to a 25% increase in pressure. As you push higher and higher in volume, a 25% or 40% increase can amount to a useful difference in tactile feel.
With respect to the 18.3 vs the 18.0, I can't claim to know a lot about the actual performance of these two subwoofers, particularly as they relate to one another. However, I would opine that a 2dB performance advantage at 120Hz is much less useful than a 2dB performance advantage at 20Hz. Why? Well if you're watching a movie on a 5.1 system with your speakers crossed at 80Hz, you could have a theoretical peak of 123dB at those lower frequencies since you're summing the 105dBx5 peaks of the satellite channels with the 115dB peak of the LFE channel. At 120Hz, you're at the upper brick wall limit of the LFE channel, and the input of the satellite channels will be much reduced assuming you're going by the THX standard 24dB/octave low pass filter for the sub.
