The Blue Tubes Analog TrackBox is an all-purpose channel strip tool designed with the ultimate in sound quality and audio character for maximum musical flexibility.
The Tube-Emulator section has the unique option of being able to switch between two different sounding tube options from Single-Triode to Dual-Triode. The independent Amount and Color controls permit overdrive of the Tube-Emulator stage to vary the harmonic content of the signal.

More info
http://www.nomadfactory.com/products/blue_analog_trackbox/index.html