Like the previous review, this pedal is likely not be what your expecting if you are look for Nashville type compression.  The tubes definitely can add some overdrive...Im just not sure if its the best sounding.  A good tube amp should give you this already, at least for guitar.  It seems Bass players are higher raters of this pedal than guitarists.  It is a bit tricky to find the best sound as there are so many variables.  It does seem to add some warmth to my hybrid amp (tube pre, trans. power).  It has not driven my MXR Super Comp out of my chain though, and is not the miraculous tone maker that some seem to describe...almost like its alchemy!  Seems reviews are over the top bad or good as they are written after first impression.  This pedal does little to affect your sound...which can be a good thing with compression.  I would like to try the Carl Martin I have heard so much about.  Seems the Keeley is rated high for the squash of Nashville.