so far, i’ve found the only solution to get a good sound - use an external USB DAC with line output, or USB Sound Card (USB headset/headphones).
but, unfortunately, while some OSs (Windows, Linux (e.g. Ubuntu), Android, even Windows 10 for Mobile) are capable to automatically swith audio output to a new audio device, ClockworkPi does not. it can successfully detect USB device, but you will need to use it somehow.
for now, i use Kodi. but setting it up on such a low-resulution screen, it’s a real pain. anyway, you can swith audio output device in the application settings, and listen to the music with a much more better quality output, than from GameShell’s headphone jack.
the next drawback, is that not every USB device will work OK. in my example, while Kodi can see USB DAC, and allows to select it, the sound can’t be routed through it. one USB headset for some reason has a really quiet sound when connected to the GameShell. and only one USB headset works great, and capable to output a nice, loud sound… but, it’s headset for talks… it’s not any good headphones with a reach basses.
and yeah, it’s a really bulky and fragile solution. but from what i know, ClockworkPi does not support Bluetooth audio devices, so external USB audio device is the only solution for the great sound.
p.s.: i don’t have headphones amplifier, so can’t check if it’s help somehow. but, so far i’ve tried in-ear headphones with impedance 14, 16 and 18 Ohms, and can confirm a lack of low frequency.