Not having done much modding or pad rolling myself, I can't really comment, but I can say that I went from the HRT HD (with AQVOX linear usb power supply) to the Audio-gd DAC-19 upgraded this year, and wow, the differences are quite important. In fact, the 'metallic treble' that you describe Kman is really a problem with a lot of DACs, even the Burr Brown based ones. While the HRT is punchy, musical, and has a good soundstage and depth, especially for its price, I think it's major shortcoming is in the upper mids and treble ranges, where it tends to sound hard.
The DAC-19 is an R2R not Sigma-Delta dac, and so it has much less noise in the chip itself, which means less digital filtering is necessary, and hence the acoustic sounds are much more realistic, and you especially notice it in the highs. They're not rolled off, and actually feel more 'present' in some ways, while of course being very non-fatiguing.
As others have mentioned, the dt150s have the potential to sound sooooooo analogue, it's really uncanny. The DAC-19 really takes them to another level, though you may lose a tiny bit of the edgy punch that some people really like with techno-based genres. For me the realism is worth it, and I don't feel that there is a lack of timing. In fact, I've been listening to a lot of world percussion music and it's unreal how many layers of drums and percussion instruments you can get with these phones and a good recording.