I literally signed up to this forum solely for the purpose of responding to your post. I can't believe what I am reading here...
First off, welcome to the forum!
About the HAP-S1, I actually have it since 2015 (minus 2 months where I sold the silver one and got a black one); and I also have the HAP-Z1ES. The Z1ES is a staple in my main audio system, while the S1 was used in my bedroom system. I chose them because of the user interface and how it can be operated without a phone, and immune to network jitter.
Anyway, back to my opinion. I didn't use the S1 much because it sounds muddy and doesn't present much detail information, and the onboard amp was just a "feature" not really a selling point. Very obvious when compared to its bigger brother HAP-Z1ES.
Then FW updated and supports external DAC and I started using it more because it's has a good user interface. Then I stumble upon this test and shared my opinion as somehow it echoed my experience. You can try with an external DAC that is "tested well" here or any DAC that you like. I ran this with Benchmark DAC2 and am very happy with the result. The amps I used is too long to list, but it does include Yamaha A-S2100 and the A-S2000, plus the Sony TA-A1ES.
The measurement is a point of reference of how "accurate" it sounds for those particular parameters. Of course, at the end you should trust your ears because this is about enjoyment not studio production which needs absolute neutral sound. IMO, for sources (DAC for example) I always look for the ones that sounds the most accurate with the least noise, because if the source doesn't present the information, no matter how good your downstream equipment is the information won't be there. It's just my 2 cents.
