Basically its like this:
AMP(High Noise)---AMP (High Noise) = Distortion due to excess current from 2 amps.
Line Out(Low Noise,Low Output)---Amp(High Noise,High Output)= No distortion unless amp is poor quality.
A line out is usually 2-3volts or less compared to an amp like iBasso PB2 which can do 20-30volts.
An Arrow 4G is about 12volts (might be wrong) and its a dinky little thing.
Imagine a PB2 feeding a PB2. Actually dont even imagine that, just makes my grind my teeth.
My car stereo is 1100w RMS (500w components and 600w subwoofer) fed by 8V preamps and the gain/volume on the 3 amps is barely at 2/10.