I'm a PA guy..
You've got something cross-wired somewhere!

Think about it..
If you are getting flute in your keyboard amp, something which is not even connected to you.

I'm guessing there's a return back from the FOH desk, possibly for foldback.

Another issue..
Keyboard goes to keyboard amp - yes?
DI out of keyboard amp goes to your small mixer, and then to stage box for PA mains - yes? (as if you are using the Behringer as a DI box)
The DI out of the keyboard amp;-
Jack or XLR?
Balanced or unbalanced?
What level (guessing line level)
How is it connected to your mixer? (jack-to-XLR, jack-to-jack, XLR-to-XLR)

If it was me I would go;
Keyboard out (stereo if required)--> to small Behringer mixer --> to PA mains stage box.
For your local foldback, use the Behringer's 'control room' outs, and send that to the keyboard amp. Would have been better if the Behringer had a separate Pre-fade aux send, but at least there is a separate control knob for the control room level.

The desk for the main PA may need to 'pad' your level.. XLR's are normally microphone level. The output from your Behringer will be line level, so a little 'hot'.

BTW - hope you are using shielded cables!

no I haven't any spare gaffer tape


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