I'm considering picking up a Mesa Big Block 750 in the near future. I have a few questions I hope you guys can answer... Is there a major difference between the two versions (looks like one has an active passive switch and the other has two separate inputs)? Has anyone had any reliability issues? (as I would be buying used) Overall impressions from those who have one?! My current amp is a TC Electronics RH450 which I really like but I've always been curious about mesa.