Crate has a wonderful acoustic amplifier was several effects such a doubler, chorus, reverb and delay. It also has two programmable notch filters to "eliminate" feedback ... point the open mike next to the amp ... when feedback occurs just press the notch filter button and the volume of feedback is reduced to about 10% ... works great but not perfect.

VOX has a Tonelab multi-effect foot board that includeds a tube for the blues sound. Switches can be programmed for favorites. VOX offers one and two expression pedal models ... to control effect volume and speed. The tap switch can control the speed of delay and other effects. The foot boards include hundreds of effects ... most not suitable for harmonca ... but does include chorus, rotary (for the Leslie sound) and harmonizer (i.e., adding the 3rd and 5th to make a chord). Put those three effects together and you've got a Hammond B-3 organ ... great effect for a small band.