I just picked up an fx100 last night. I have a Fender Blues Jr. NOS. I just went from the fx100's Amp Out to the Blues Jr.'s input (no effects loop on the Blues Jr. or on the Peavey Vypyr Tube 60 that is my other amp). I turned the master volume on the amp up pretty high, and the volume (gain) down to about 1. I was guessing this would color the fx100's sound the least. I just set all the tone controls to 5, though I inuitively know that is probably not optimal. Regardless it sounded okay for a first impression. It could be better I'm sure.
After figuring out that streaming audio doesn't go out through the fx100's amp out, I tried the fx100's main out to the front of the amp, but I didn't like that at all, at least with the initial settings. However, that was just the impression with streaming audio--it sounded so bad I didn't bother trying the guitar that way. It was just really thin sounding. It was getting late though and I was really tired so I might be able to get that to sound better with some tweaking.
With all that said, I may just grab some KRK Rockit powered speakers. I had some a while back and they sounded pretty good with the HD500 (sold). Either that or I'll just use the headphones for a while. I don't feel like running cables across the room from my guitar area to the entertainment center.