I'm comparing the measurements at the top of this thread for the s9 with those for the
s8. It looks to me like the s8 has slightly better SINAD of 114 dB (vs 108 dB for the s9). The s8 has a dynamic range of 121 dB. I don't really understand why there are two dynamic range measurements for the s9 (118 dB and 79 dB) both labeled "balanced out", but that second one is considerably worse than the s8. The s8 also looks to have better jitter performance. Multi-tone performance looks about equal.
About the only thing that looks better for the s9 is the power, which on the flipside, likely has a negative impact on battery life.
Since I'm really only planning on driving IEMs (Etymotic ER4SR) that don't need a lot of power, I'm not seeing any reason I should get the s9 even if it were the same price. Is there anything I'm missing?