If i remember right, you can setup the MF trem algoryth to be triggered from an expression pedal to do a volume sweep. I do have a volume sweep preset on my MF that I use. The problem I ran into was with my softstep. When I limited the sweep within the softstep, as to not have the volume not drop all the way down to zero, the pedal did not compensate to have the lower volume start at the full depression of my ex pedal. At the point, I became sidetracked by another project.
I do see the benefit of having a dedicated volume swell algorithm. I would put that on my high priority purchase list.
Maybe the algorithm could include many juicy possibilities. I could dream up many musical ideas, such as the ability to dynamically color the swell with eq and/or time based fx, that would make the swell very multi-usefull.
