SWR Interstellar Overdrive. Took me forever to find mine, and I paid too much for it, but what a piece of kit. The real secret is that while it does an amazing tube distortion for bass, the clean tones are just incredible. That said, I run mine through a Marshall EL34 100/100 power amp, and with the tone wide open on a set of EMG Geezers, the sound is planet smashing.
The main drawback is that mine is a sensitive piece and the distortion seems to change depending on how hot the tubes are. This means it requires constant adjustment and isn't very good live. For practise or recording, though, it's unbeatable.