Topic: Dim headphone output

Totalmix FX with Fireface 802. When talkback pressed on ARC, Is this possible to dim specific output ?
My problem is if I monitor a headphone mix (ex: output 5/6) Cue through my speakers in the control room, and I press talkback, I got feedback !!! Mic talkback goes to output 5/6 (which is monitored with the speakers) then to talkback mic.... got the picture ?
Any solution ?? thank you