Cream Tube is based on one of the most famous overdrive circuits ever. This circuit was used in different versions, but the tone remained pretty much the same through the variations. The reputation of this effect is due to the general design and gain stage specificity.
The general design relies on the succession of three amplification stages, each providing some clipping. The middle stage is the main Gain stage. That stage was designed to give a smooth overdrive tone close to tube sound, using only solistate components. This stage clips the signal but let a specifically filtered part of it pass below the clipping treshold. The result is a “clean distortion” sound that gives this effect a unique place in the distortion history and makes it a great addition before any amp.