It depends what your wanting to do or what you're using the Boss for??? Heres a couple of ideas:
* If you run an FX processor in to the input of the amp, you are also using that amps preamp along with your fx processor and running the FX through the preamp. This is what I do when I run my PODxt in to my old fender deluxe, the podxt adds distortion and/or effects to signal before it goes to the amp.
* If you are using an FX processor in an FX loop (send and return) you are sending the amps signal through the fx processor and adding FX after the preamp stage. Unless the amp has a paralell fx loop, then it actually has two signal (one dry and one fx'd) and you blend the two - I really dont like paralell fx loops myself because you cant really use any modulation fx (like flange, chorus, phayse) in the fx loop, thus you have to use a pedal/fx unit in front of the amp for this.
* On amps with a serial fx loop, you can run into the FX return only from something like your Boss and use the amp as a power-amp and your Boss would be the only preamp.
Man, I just confused my self :shock: Maybe some of this will help, or someone might give you an easier answer.