MOTM

I have a TubeWorks Blue Tube that I use for everything from warmth to crunch. 
 There is a two-channel rackmount version that is quite nice too.

One thing you have to watch, especially with high-output devices like MOTM, 
is that you keep your input level to the unit fairly low so that you can 
maximize the range of sounds within the device.  Otherwise everything will be 
massively distorted (sometimes good).

Ivan