Don't confuse the change in acoustic sound pressure (in Pascals) with perceived loudness curves (more on that below).

Once dB is converted to Pascals (which is a linear function), the percent change in acoustic sound pressure can be determined.

Baseline = starting sound pressure level.

+1 dB = 1.12X baseline

+2 dB = 1.26X baseline

+3 dB = 1.41X baseline

+4 dB = 1.58X baseline

+5 dB = 1.78X baseline

+6 dB = 2X baseline

+7 dB = 2.24X baseline

+8 dB = 2.51X baseline

+9 dB = 2.82X baseline

+10 dB = 3.16X baseline

+11 dB = 3.55X baseline

+12 dB = 4X baseline

Say for example you have one subwoofer being measured outdoors and the SPL at 30 Hz is 90 dB. If you took a second identical subwoofer and place it next to the first one, and also set that subwoofer to 90 dB at 30 Hz, and then turned them both on, they resulting SPL would be 96 dB. So a 6 dB increase in SPL is literally a doubling of the acoustic output. This can be verified by converting to 90 dB to Pascals and 96 dB to Pascals and you will note the 96 dB Pascal value is literally 2X the 90 dB Pascal value. Similarly, if you took two more subwoofers (for a total of four subwoofers), all operating individually at 90 dB @ 30 Hz, the combined SPL would become 102 dB. Again converting to Pascals, 102 dB is 4X more than 90 dB in Pascals.

If you take a look at the equal loudness curves, the old '10 dB = a doubling of perceived loudness' only hold true over certain portions of the audible spectrum, and equal loudness curves are expressed differently than actual changes in SPL, so don't confuse them - more on that below.

You'll note that the equal loudness curves compress in the deep bass regions. So while we're less sensitive to deep bass frequencies (meaning the hearing threshold is higher), once that threshold is exceeded, wer'e actually much more sensitive to perceived loudness changes than we are at higher frequencies.

Finally note the equal loudness curves are expressed in sound intensity and not sound pressure, and the two units are not the same (the log10 functions are different), despite the fact they are both dB scales.

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/eqloud.html