You know you have to bias the tubes and tinker with them and replace them when done with their life...Bada headphone amps have really good reputation for sound quality, but they break down easily.
If you want an upgrade from NAD, try Teac AI-1000 or even 2000 on ebay I know Teac is not the name you associate with hifi, but these are gems - they have parts found on much much more expensive gears, and sound like it too.