I've been using my M50's for a few years now, and recently got in on the HD598 $100 deal over black friday. My initial impression of the 598's were that they were a little dull, but I realize now that is kind of relative to the M50s. Anyway I attributed this dullness to the Sennheiser Veil I keep reading about.
When I first used the Fulla, that dullness was gone (could be due to the heightened Treble that has been mentioned in a previous review). There was definitely a great amount of clarity than my headphone jack. I compared the two quite a bit, and the Fulla was the victor in my non-blind tests. The imaging and clarity were the two things that stuck out most.
I had tried the Fiio E17 previously with my M50s, and was rather underwhelmed, ended up returning it since it was buggy. This was my second attempt at trying a dac/amp and I do believe it was one of my best purchases. I haven't experienced this level of euphoria since upgrading from $10 headphones to the HD280's. I was riding cloud 9 all Friday night. I look forward to upgrading to magni/modi and Valhalla once I upgrade my headphones.
I'm just starting out in this hobby, but I'm amazed at what this little Amp/DAC has done for my 598s.