I sent this the same mix to two different mastering services. Compound Audio was one of them. I did this to further narrow the gap in my mind between the
subjective and objective nature of the mastering process.
Compound Audio simply did a better job of demonstrating what I can only refer to as exposing a greater "depth" in the audio while preserving the my tone, also showing a smoothness in the top-end which was perfect for pressing the loudness.
My expectations are higher, I have confidence in my mixing targets, and I'll be working with them again. You learn from being exposed to quality - that's it.