An objectively "good" amp, as I would describe it, is the one in the LME49600 application note. Quite a few commercial and kit designs use variants of it, and the original uses a top-spec (LME49200) op-amp and has a DC servo. Powerful, low distortion, low output impedance, and devoid of the headaches inherent in discrete semiconductors (which only outperform monolithic chips if you've done some very careful hand-matching.) And not too expensive, either - I think it specs out at $30-$40 in components, and quite a bit less in quantity.
Same thing with the ODAC. Low jitter, low distortion, low noise, not particularly complicated.
As far as I can tell, the difference between the two is a Corvette -vs.- Porsche thing. The Porsche has better looks, better fit and finish, better feel, better seats, better noise insulation, and a much more storied history than the plastic car from Kentucky. But put them on the Nurburgring, and a $48,000 Corvette can keep pace with Stuttgart toys at twice the price.
Nuforce and Schiit appear to operate under the old-school "fiddle with it until it sounds good" school of audio engineering. This often doesn't produce ideal results, especially with inclement operating conditions, and often omits things like protection circuitry.
I'm not anti-luxury. Far from it, in fact - I'm sorely tempted by some secondhand Audeze LCD2s, which are the first headphones I've ever seen to post results that can seriously compare to high-end loudspeakers. However, while fancy engineering tweaks are still required to make a transducer produce a respectable square wave, a $3 op-amp does it just fine.
