HA-INFO U2 Plus
Great little USB DAC, with impressive specs for the price which is £45 ($70):
Tenor USB TE7022
TI 6120A2 headphone amp
Good quality caps
After replacing the stock op-amp with a OPA2134 and using Sennheiser HD600 headphones (300 ohm) the sound is smooth and dynamic but controlled and with good detail. There's also enough power to drive the Sennheisers to fairly high volume levels.
Despite their high impedance, there is very good synergy with the HD600s; however I can't vouch for other headphones, as there is the possibility that this unit may have a high headphone output impedance, which means low impedance headphones - which are usually most suitable for this type of amp because of the ease with which they can be driven - will may not sound as good. However this is speculation on my part, other users may be able to confirm or deny this.
Despite this reservation I'm surprised how this unassuming little unit can manage to sound so authoritative and musically satisfying (with my headphones) for such a low price.
One thing worth mentioning is that the HA INFO needs at least 60 hours 'burn-in' to start sounding its best. At the beginning when plugged in from new, the unit sounds quite hard and cold. After the burn-in process (I gave it 90 hours in total), it opened up nicely.
Edited by Mr Vicarious - 1/3/16 at 5:43am