Universal Audio M610 by brianellefson
I've owned this pre for about 3 years now and it gets extensive use on vocals. Really warms up every source quite nicely - takes the sterile sound out of things.
Bass and Vox sound best on it. I've tried acoustic guitar and it just didn't quite fit. Gain controls let you dial in the amount of "tube" sound without overloading the signal. My only issue is the size of it considering that it's just ONE pre and doesn't house a compressor. UA has since discontinued this pre and sells the Solo/610 in it's place, which is smaller but appears to have most of the same controls.
Great sounding tube pre. Best on vocals and electric bass guitar. Takes some experimenting on settings to dial in the right sound for the source, but the nice thing is that it HAS the capability of dialing things in (Hi-Z input, selectable impedance, phase, etc.)