Mesa Boogie V-Twin Rack Two Channel Tube Guitar Preamp
Hard to find Mesa V-Twin Rackmount Preamp.
This is an entirely different animal to the pedal version as the tubes fun in full voltage and it has a much more developed circuit.
Entirely tube driven, with the original Mesa branded tubes
Two channels, clean and dirty, switchable, and with three switchable sounds within each channel and separate EQ sections for each
Reaches extreme levels of sustain and saturation if desired! And unusually the clean sounds are also great. 
Speaker sim output and regular output
The Effects loop volume pot needs to be replaced.  I've never used as I was using this into a mixer and the unit works fine without the loop.
This unit runs on 230V so suitable for UK and European use.