Old thread but just in case anyone is working with a Flextone III + an FX100 or a Behringer Keyboard amp.
I was unable to get sound with Uber Guru's 2nd method until I put an unwired adapter plug in my Fextone.
A great feature of using the Behringer Keyboard (KX1200 - 3 way with 15" woofer) was that the loop (1st method only for the Behringer) permits me to use the one FX100 with another guitarist (or two). I can also change the mix of effects on the Behringer to have a totally clean sound (which is pretty good on that amp IMHO). The sound when looped was not as good on the Behringer as going direct, guitar -> FX100 amp out (not main) -> Behringer, I really like that sound chain the best.
The sound with method 2 through the Flextone was good, but for sound quality I preferred method 1 with my four amp channels set to different amp models on the Flextone (all with drive dialed back)- I get interesting different flavors of my FX100 this way.