I have a DAC2 and a STI-1000. I have tried using the Dac2's pre-amp in HT bypass on the STI and something was missing.The DAC2's sound was not bad but the STI's was superior. I prefer the STI's pre-amp section to the DAC2's.
The pre-amp in the STI is very different than what is built into the DAC2.
Of your choices I would buy the DAC1 and the STI because the DAC1 will probable be long gone because of advances in technoloy when th STI will still be great integrated amp.