This vocal preamp is perfect for adding true tube warmth and shape to any voice. It is extremely simple to use and set up (like many Electro-Harmonix products). It's input gain knob acts like one would expect, increasing the gain on the signal BEFORE the tubes. This allows you to add any amount of warmth, from classic clean to raspy overdriven. The output gain can bump up (or down) the signal after the tubes.
My only disappointment with this unit (if it can be called that at all) has been that the output gain is limited in its ability to control the whole signal. I think this is more of my own doing than the unit's. Specifically, I leave it in the "off" (all the way left/counter clockwise) position and it still puts out a signal at quite a high gain level, thanks to the input gain. I run the input high into the tubes to get some crackle and ambiance, so the output has its work cut out for it. If you are playing a venue, this unit has more than enough gain to get the vocals above the whole mix. I would still like the output gain to work in more of a volume control fashion than true gain, but it certainly demonstrates EHX's understanding and mastery of audio equipment.
This is one of the few units that can be purchased, plugged in, and instantly does what it says.