Any thoughts on a good pedal combo for electric sousaphone?
If you're looping, my suggestion would be to get pedals that allow you to get timbral diversity and greater frequency range. [on trombone] I get my timbral diversity by employing a couple mutes and some extended playing techniques, but you can also get a lot of mileage from delays and filters.
Delays are quite versatile and can produce various classic effects (phasing-like, flanging) at lower delay times as well as providing you'd expect from a delay. Look for a pedal that has delay time modulation for greater possibility.
I use a lot of playback rate adjustments to get trumpet-register sounds into my loops, but you can get very good results with a pitch shifter like a digitech whammy (have a listen to this guy on contrabass flute: http://youtu.be/BiqPn3d_dKM )
Additionally, you may find it worthwhile to investigate harmonizers. A brass instrument harmonized in fourths or fifths can sound quite good.