Hi CyberGhost, I can't tell you the difference, but I can tell you how it sounds with Grados. It's not a great match. I have the Grado RS1i and just bought (and returned) a DF 1.2.
The Grados tend to emphasise mids and treble rather well - some would say too well. Through my iPhone 5s and iPad 4 I have rather enjoyed the sound with rock music, actually most music. I think they are fabulous headphones.
If you believe HeadFi you won't get the best out of your headphones without an off board DAC. So I ordered a DF 1.2 and received it last week. I've used it primarily with my Grados, but also with TPeos H200 IEMs, mostly on my iPhone and iPad, but also on the iMac. Yes it improves clarity a little, and there seems to be a little more ambience and depth. Bass notes have better definition. However, to my ears, it does this by removing some lower mids and bass. On headphones with plenty of bass this could be considered removing the flub and mud. On Grados though, it just sucks out too much mid range and low end. Bass guitar is crisper, but brighter. Treble is brighter, often piercing. Guitars sound brighter and thinner - losing too much mid range and warmth. I found it tiring to listen to. It sounded a bit scooped. Something like Norah Jones Come Away With Me sounds lovely - detailed acoustic guitar and nice vocals; but Nightrain by Guns N Roses sounds too bright, too thin. Ironically, both sound lovely without the DF, just Grado straight into the 5.
The results were better with the H200s, though very subtle. They have a thicker sound to begin with, so the scouping seems to have the desired effect of cleaning things up, allowing you to hear more detail. The difference between with and without the DF was very subtle though. I probably preferred with for the H200s - the midrange and bass seem less congested, the treble seemed cleaner.
In summary, for Grados, I felt I wasn't gaining a significant amount of detail or staging, it was subtle, but there. However, I was losing too much mid and low end - they became shrill, thin, lacking in body and warmth. The H200s sounded better, a little, but I'm not sure they felt like £129 better.
I posted mine back today.