I did a web search and found that Stereophile had done a review of the Zu Soul in 2016 (they also reviewed your amp in 2011). This was a review of the Soul and not the Soul 6, but the two appear to have similar woofers and may well have quite similar specs. The manufacturer states the Soul has a sensitivity of 97dB/2.83/m. The Soul 6 they rate at 100dB.
Some here on Audiogon are skeptical of John Atkinson's measurements, but I believe him to be unbiased and that over the years he has created a remarkable and consistent data base of speaker and amp measurements. As is often the case with speakers whose manufactures state a sensitivity over 92dB, his measurement came in at 91.5dB. Again, this is for the Soul and not the Soul 6. Assuming that the 6 is similar, I'd say there is a fair chance that your 12W amp is a bit under-powered for the speakers. Additionally, many of us notice some edginess in the upper mid-range when pushing our systems.
In general, it seems that very few speakers with front loaded woofers achieve sensitivity above 92dB. To honestly get to the 97 and above range the woofers must be horn loaded, such as the Klipsch La Scala.