I got fatrat as a fan of '80 rat sound and i was really curious how the new features (fat & mosfet) could affect the sound of my rickenbacker 4003. Well, the result is really awesome. The "fat" mode is the way to go for the bass. The "mosfet" mode makes the sound more wide and soft. though I found out that the "stock" (not "mosfet") keeps more the attack of playing. Depending of the needs i switch it to "mosfet" but i mostly play with "stock". Also, it is remarkable (as in all rats) that with just 2 knobs and a volume knob you can make out various sounds, from clean with little dirt to real distorted and fuzzy. A very very interesting thing is that the op amp is socketed. +1 to the open minded people of proco for adding this option. So, if desired, the replacement of the op amp with others is really easy. I did change it with the legendary lm308n and believe me it is worthy doing this. The sound with the stock op amp is really good but lm308n gave this pedal really better fuzzy low end.