Reason for worsening audio quality in a Pi-based audio system?

I have bought a DAC+ to put on my Pi, and directed the output to a Topping NX1 portable headphone amplifier. The sound quality of the system feels amazing, and I have put it in a metal case (with a drilled hole for the audio jack) and linked it to a portable power bank, to carry it around with me as a portable music player.

I have been using stock Raspbian, and use ssh client on my phone to control the Pi. Recently I have decided to try out Volumio, so I flashed a *different* SD card, and booted into the system. However, although Volumio supports hifiberry+ driver and alsamixer, and plays all lossless formats, for some reason I feel the audio quality seems to be not as good as what I hear in stock Raspbian. So I switched back to my old SD card with Raspbian.

But the problem is...later I have gradually noticed that the sound quality of the system is not as good as before, when listening to the same audio files. Especially, the power of bass has dropped visibly and is now feeble and a bit muddled...and the soundstage seems to be not as wide as before.

I'm not sure the problem happened to which part of the system...I wonder, if there is any possibility that the Hifiberry sound card has some onboard memory for configuration that might be changed by the Volumio system, which might cause the sound to change even after I switched to a different SD card? (I see in the specifications that Hifiberry DAC+ has onboard EEPROM for configuration, but it is not fully functional yet?)

Also, as I carry around the DAC+ and Pi in the metal case in my pocket a lot (but the boards are attached firmly to the case with screws, and spacers on DAC+ are mounted), could the board be susceptible to damage in someway because of vibrations or overheat? Will there be some way to check if it is working correctly?

(But of course...this could also be a problem with my amplifier, which will affect bass and soundstage a lot. However I have mounted the amplifier to my old devices (phone, tablets, etc) and it seems that it's working fine...)

Thank you very much!

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