To understand the merit of this thing, you have to realize it is NOT an amplifier. It is a buffer. Meaning, all it does is pass a signal through a tube with almost no appreciable amplification. The catch is that it is preying on people thinking that simply having a tube in the chain is what makes the sound better. That's not the point. "Adding warmth" or "adding color" is not the point. The point of a tube - what makes it better for audio than a transistor - is the way a tube distorts when it amplifies a signal. Yes, the even-order distortion produced by overdriven tube circuits is more musically pleasant than the odd harmonics produced by an overdriven (clipping) transistor. (you can google using terms like "tube transistor distortion odd even harmonics" to better understand this)
If you're not amplifying the signal with the tube, there is no point. You're just adding the distortion inherent to the tube to whatever else in your chain you're already using to amplify. So if you're not REPLACING a transistor with a tube for the various steps in your signal chain, you are not gaining sound quality, period. In the case of this thing, you're also adding the distortion of an extra volume knob, and the bass and treble controls.