Keep hearing plugged instruments through headphones when volume knobs are to zero.

Hello.
When I plug an instrument to any of the inputs of the Firestudio Project, I keep hearing it through headphones when its physical knob is turned to zero.
I usually send to the Firestudio already processed signals (pedalboards, amps etc.) but trying to figure a way out of this issue I noticed that by plugging a guitar directly to the Firestudio Project and using software effects (Garageband/Logic) what I monitor on the headphones are both the processed and the direct sound (clean).

I guess there’s something wrong in the Universal Control settings but despite my efforts I caot resolve the situation which is quite frustrating.

I’m struggling with this issue since I’ve replaced my Firestudio Project with another identical one after an electric failure, months ago.
Before of that I always bypassed the Universal Control by turning off all the output pairs 1/2 3/4 5/6 7/8 but now it doesn’t work.

Hardware:
Firestudio Project
Macbook Pro mid 2010

Software :
OS 10.9.5
Garageband 10.1.1 /Logic 10.0
Universal Control V 1.7.4.5

What I did:
Searched info on the user manual
Reset the Firestudio Project to factory default
Updated the Firestudio Project firmware
Messed up with Universal Control Mics and DAW without success.

is there a solution?
Thank You.