Thanks, Andre, for the detailed explanation!!
I don't think your Denon example is a good one, though, because that involves DACs and power systems. Obviously there's no limit to how good those can be. But in my application here, I'm just streaming an MP3 onto Bluetooth, and I don't want to apply any processing. I'd imagine that the digital decoding and re-encoding should follow standards, and give the identical result as any device following the MP3 and Bluetooth coding standards.
On a separate topic, in my car I've been using iOS app Equalizer by Audioforge in order to flatten the response of my speakers and boost the bass. I'm blown away by how well it works. And it also doesn't support browsing by Genre->Artist, a significant disadvantage! I don't know about you, but I'm usually in the mood for a particular genre, so it makes total sense to browse artists within a desired genre.