NUX won a Best in Show at the past Summer NAMM, the fact that Mike Hermans picked one of their pedals for his video series is also very promising: here’s his demo of the Tube Man MKII Overdrive – quite obviously, a Tube Screamer clone.
More gain! That was the overall goal. If you’re not playing through a real tube amp, cranked for overdrive, it’s difficult to get genuine tube overdrive sound.
The original Tube Screamer pedals used an OP-AMP system, a basic circuitry that became popular for its “perfect amp” attributes. It had no distortion so it had to be overdriven or distorted by re-shaping the signal until it distorts ( diode-clipping ). Clipping it a little bit gives you Overdrive. Clipping it a lot gives you Distortion.
Using an FET Transistor System, which already has overdrive and distortion capabilities is a better approach to maintain the genuine sound you get from an actual analog tube amp. An FET allows you to bring out the harmonic attributes, and break-up the distortion into a finer grit, the way a real tube does.
More info here.