Volume issue

Yesterday i installed the new JY-UQS03N4G Headunit in my VW T4. The Rear Speakers (Crunch 4Ohm) are directly connected with the Radio.
The Frontsystem is connected over Hifonics 4Ch Ampflifer on RCA Front L/R Chinch Connectors.

My Problem is that the Volume is extremly too weak for this unit, i set volume to highest (36) and i hear clear sound but is many too silence for me.
My old chinese Android 7“ Unit was many more louder than this headunit.

When i fading only to the rear Speakers i hear sound only betwen 15-36. The Volume are very weak. Same over RCA Outputs, the levels are the same too weak for my amplifier.

Is there an option to increase the Ampliferoutput? I didnt find anything in Factorymenu. Im not satisfaid with this, i installed over 4 hours and now this. Angry