IMO, it is not just about whether to get a pre or not, is more about the numbers. Your dac outputs 2 v on the single ended output. Even though Aero sings the praises of their dac serving as a pre, that output may be too low. I can share my experience. My dac (benchmark) can function as a DAC/Pre or just a DAC. When I got a preamp, and used the DAC in DAC only mode, the sound improved for the factors you describe are lacking. My DAC also has a jumper to output three different output voltages. I have tried each different output level and I found the highest level sounds best, even though traditional thinking is that such a high output voltage is less desirable. The sound is more full, bass is better, and the soundstage improved.
The fact that your analog input to our amp is higher and sounds better aligns with a theory that a higher input voltage into your amp will improve the sound. Can you try xlr balanced input/outputs? That will double the output voltage of your DAC. There could also be some impedance matching issues but I have not looked into the numbers on your system. Finally, it could just be the differences in your system between digital and analog. Do you find the digital system more detailed, which is to say, are some aspects better? Please keep us updated.