Most DAC/amp differences are very subtle with most headphones. The main differences in DACs from what I've experienced are differences in the low-pass filter (and those differences become less important the higher the sample rate), and I've read about differences in DACs stemming from how they handle intersample overs in the music (which you can avoid by setting a negative digital preamp gain).

You experience is not unusual, there's like a post every week about how some person can't hear a difference in DACs that they have. Now that you have experience, you can make choices accordingly. Since you know that you don't hear any difference in DACs, you don't need to spend more money on them unless they have some feature that you want!