I don’t know any audiophiles period. Hence the lack of knowledge on the subject.

I have a couple different sets of speakers and know that my larger ones sound better than the smaller ones (more “tinny” sound), but figured the core difference isn’t the quality of analog output on the source. Rather it’s the larger amplifier and larger size for bass generation.

For Bluetooth the problem is different. While most people will probably not consciously notice the poor audio quality after long hours of listening they will experience fatigue. This is less likely with lossless audio.

I have a 2015 rMBP and its audio sounds as good as through my CEntrance DACport Slim. Maybe my headphones aren't audiophile quality (Grado SR80i, Sony MDR-7506)?

It's possible. I haven't used the CEntrance DACport Slim, but it looks promising. After looking it up I would set the DAC to the high gain setting and drop the volume lower.

And the Grado's look like a good set of headphones.

I would try listening to The King's Singers, which is an A Capella voice group that sings classical music. I can pick out individual voices in space from recordings that were recorded in the late 1980's. A good DAC won't smear them in space, and you can "lock on" to a voice and hear the tonal quality of it individually as if they were standing in front of you.