I like the UDH-100 very much and it is much much better than the Icon HDP. with my AKG K702's using the UDH-100's amp 75% on the volume knob suffices 80% being loud, same with DT990. I have not used my Schiit Valhalla for a long time but they are about the same but not really any better no. the Schiit Valhalla and UDH-100 are completely silent and work just as great.
The USB on the UDH-100 is SUPERB. there is just driver issues that Nuforce is fixing. yes the UDH-100 is very much better than the HDP. as for power well... I always have my UDH-100's analog out hooked into the AUX of a vintage Kenwood KR-7400 and that drives headphones like crazy it really makes the drivers move, the UDH/valhalla does not even come close in that regard.
UDH-100 is worth it.
Thanks for your answer. I'm kind of skeptical now to be honest. Having to turn the knob to 80% to get headphones loud is never a good sign about the amp. I would not like to pay so much money for it and then to find out some music thats not recorded very loud is too silent and I cant get enough volume out of my headphones. In terms of pure headphone output power, is the UDH weaker than HDP? Because I've read somewhere that HDP even runs HD800's or HE-5's properly, and has plenty of power for K702, thats why I'm surprised to hear about UDH's amp... do you ever feel like its lacking power to get the K702's really singing very loud?
How is the sound signature of HDP and UDH? I heard HDP is pretty cold and analytical, is UDH any warmer?