Clarion GL-300 has preamp outputs for 2 channels and a subwoofer, but its RCA output voltage is not consistently published in the specifications. This is common for unbranded Android-based head units, where the specifications can vary between different production runs. 
Pre-output and DSP details
According to several retailers' specifications, the Clarion GL-300 includes the following: 
Pre-Outputs: 2 speaker outputs and 1 subwoofer output for connection to an amplifier.
Power Output: A built-in amplifier rated at 4 x 45 Watts.
Built-in DSP: The unit includes an integrated Digital Sound Processor (DSP), which has a multiband equalizer to optimize audio output. 
How to determine your head unit's RCA voltage
If you need to know the specific voltage for your unit, you can test it directly: 
Connect your head unit to a power source.
Set a multimeter to measure AC voltage.
Play a low-frequency test tone (around 50 Hz) at 0db, which can be found on car audio websites.
Increase the volume on your head unit until the signal starts to clip (distort). You should see a reading on your multimeter.
Check your amplifier's input voltage requirements and adjust the head unit's gain settings to match for optimal sound without distortion.