I've got the MXR ZW overdrive (JFRICK also does), and I love it with everything I use it with! It will really push a tube amp/preamp to the next level of sonic nervana. It also has a really nice Marshallesque crunch when driving a clean channel, or it can give you a subtle bit of overdrive for some sweet bluesy tones. In front of an already overdriven channel, it can really tighten things up and rip your face off, with any level of hard rock to full on metal gain you could desire.