I always think it's funny when people refer to the Grace as being warm and smoothed over. I'll agree that it's a very smooth sounding amp, in that it's not harsh, but I started my headphone journey with HeadRoom gear and comparing the Grace to the HeadRoom stuff is like comparing the RS1's to HD650's. Oddly enough the first thing I thought when I tried the Grace after being used to my HeadRoom amp was, "woah where's the bass?" Now that I've borrowed the 902 for about a week I grown accustomed to its more separated cleaner sound, however I still feel it sounds kinda analytical, which I know isn't the case according to most listeners here. In light of that, if the D1 amp/dac combo is considered to be a fair bit more clean and analytical than the m902 I'm not sure if this is the type of product i'd be looking for. I like stuff with really punchy deep bass and liquid mids and highs. My favorite listening experience to date as far as frequency response goes are still my recabled UE TF10's. I realize everybody's ears are different but I figured as a music minor in college that my ears would be somewhat neutral, however I'm slowly discovering this isn't the case. I've heard the DAC1 once and thought it to be incredibly harsh.
On a different note the m902 uses a transimpedance amplifier design which is apparently designed for higher current low impedance headphones. My Grado's sound fantastic with the m902 very punchy bass and highly dynamic (of course they're known for their impact). The HD650's on the other hand sound very flat and boring in comparison. I'm not sure if it's the amp that lacks power or if the headphones just only respond well to warmer bassy amps (a lot seem to like the HD650's with tubes). All in all I'll probably end up either selling the 650's and getting a pair of RS1/RS2's and buying an m902 or testing the D1 for the 30 day trial.