I also have the DAC2 directly driving the ST-500 via balanced. I think you may need more than 100 hrs for the DAC2 to burn in. Also after some time you can tweak the STI by changing the two fuses on the modules. There is one on each. I changed mine with the PS-audio fuses and got smoother yet very detailed upper end. From my experience also a good power cable later will benefit both the amp and DAC2, specially the DAC. I changed mine nearly after many months but I got a big step up in soundstage expansion.
Good luck.
Good luck.