One thing to make sure of is that the volume pedal you choose has a high input impedance. I got a passive boss volume pedal awhile back and have had to set it up with special cables to keep it from messing with my input impedance.
The worst thing is, the di on my pre-amp still sends a full signal to the desk. The end result: my arco sounds right to me out of my rig, but like a buzz saw coming out of the p.a.
I think I'm going to have to sell the pedal on and get an active one.