Big wet and splashy. 60’s outboard spring reverb tank.
Their goal was simple make a reverb pedal that functions just like a real deal outboard spring reverb unit. They felt that adding more knobs for the sake of adding knobs would take away from the experience, so we kept it down to the controls that were on the actual units. They only added or extended controls that contributed to the experience in a positive way. The DWELL knob controls how much signal is going into and driving reverb springs. As on the original unit, cranking this knob up can cause clipping on the wet side due to the signal crashing the springs. Though it seems to imply reverb length (and sort of functions this way), this control is quite literally a volume knob for the springs.
The TONE knob allows you to roll off just the right amount of high end from only your wet signal. Roll this knob back if you want the reverb to appear to more distant and out of the way of your dry sound.
They took liberties with the MIX knob extending beyond the original reverb tank units because they wanted you to be able to make the Topanga 100% wet. Otherwise it is much like the knob on the tanks, allows you to balance between your wet and dry signals.