The relation between voltage and SPL is logarithmic rather than linear. A 2% decrease in voltage level gives only a 0.2 dB decrease in SPL (20*log10(0.98) = -0.1755 dB).
Also keep in mind that the maximum output of the S9 is given as the RMS voltage for a sine wave. The peak voltage is thus greater by a factor of square root of 2, i.e. ~1.41 or 3.01 dB, i.e. peak voltage is ~1.4 V. This gives peak SPL at max output of ~2.8 dB above sensitivity at 1 V, i.e. 113-114 dB.
Depending on the music, average levels could thus be well above 100 dB SPL. As discussed above, however, the frequency distribution of the signal is crucial in determining how loud this is. A 30 Hz tone at 114 dB SPL is as loud as a 1 kHz tone at 80 dB SPL (80 phon) whereas even a 40 Hz tone at 114 dB SPL is as loud as a 1 kHz tone at 100 dB SPL (100 phon). Further, 100 dB SPL average at 50 Hz is as loud as 80 dB SPL average at 1 kHz whereas 100 dB SPL average at 200 Hz is about as loud as 100 dB SPL average at 1 kHz. The former, I would still consider only loud while the latter would be very loud indeed.
Source:
https://en.wikipedia.org/wiki/File:Lindos1.svg