Constant bit rate is just that - constant, whether it's needed or not.
It wastes data when it's not needed & it can't keep up when it is.
In areas of low complexity, therefore, CBR is just spinning its wheels not really doing anything. In areas of high complexity the rate isn't high enough to capture all the detail.
VBR avoids this.
CBR was useful in the ...
An external ADC/DAC chain might sound better than if you did it in the PC.
Is it really? Possibly, or it could be apophenia or other psychological improvement. You would need to do controlled testing with many people to be able to say what actually happened quality wise.
Assuming your ripping from lossless source to Opus/MP3, Opus is definitely superior sound quality as this is shown in listening tests from Hydrogenaudio website.
as a general rule... Opus/AAC(standard AAC-LC)/Vorbis(.ogg) etc are superior to MP3 in terms of sound quality simply because they need less bit rate to achieve a certain standard of audio quality. ...