Brooko posted a review of the Hybrid Valve amp on Head-Fi. This is one of many portable tube amps which have hit the market recently. The one nice thing is that this is a combo DAC/Amp with a cost of only $179.
Pros: Transparency, build quality, portability, use with iOS (special cable required), DAC resolution, sound quality, aesthetic appeal
Cons: Valves distort if over driven (line in source needs to be variable), low powered for higher impedance cans, no way to turn off USB charging.