yeah the build is better absolutely, and the gain is marginally better than a D10 with higher outputting opamps due to the higher powered battery. but IMO there are many reasons to buy the D10 over it. the price being only one of them. the D10 accepts higher resolution input via optical and coax, rather than relying on up-sampling to take 16/48 to 24/96, its much more flexible and provides a true line-out function for driving other amps, is tunable via it opamp rolling and has support like I have never experienced before. I sent my D10 back for a service and even though I had modified it, because they saw I knew what I was doing, are still fully supporting and guaranteeing the product. they also installed the latest lot of updates to the power rail and coupling caps, updated the input and output jacks (bar the coax input), gave me the new volume knob and basically gave me a new version D10. I have since installed Blackgate caps and they are still supporting me. the D10 was returned back to me after these upgrades and also running a battery charge/discharge test within 4 days of them recieving it. thats fixed, upgraded and shipped to australia from china in 4 days!!! this sort of efficiency is why China is taking over the world.
@ OP: the P-51 would power your headphones, but would not give you a dac, so you would still be stuck with a crappy source. do not get caught up in the amp hype, portable amps do make a nice improvement, but they will not provide quality sound that isnt present in the signal in the first place.