Hi Hapster. Thanks for your post. I underlined a couple of things above, because the way you've worded them suggests that you haven't abx'd the two formats properly (level matched and blind).
It's entirely up to you if you want to spend the time doing it - and all it will help is your own personal knowledge. I do think it's worth it. Most of the people who invest the time to test themselves admit to it being an "eye opener". If anything it lets you know your own limitations. It'll probably also show that you are one of us (ie normal human beings).
Here's a link on how to do it. The software is free. All you need is access to a PC, the internet, and a CD.
Important things are using the same source file for comparison, volume matching, and making it blind (eliminating placebo).
So I wasn't completely wrong.
The bad news: There were no "revelations" in the song, it appears that the 320kbps was a "digital version" if you will, and the FLAC came right from the CD, so transitions were different, which is basically the "new sounds" that I was hearing.
The "Good" news: I was correct 18/20 times when deciding which was higher quality, now my theory is if there is no distinguishable difference between 320kbbps and FLAC then it could be entirely up to the fact that the CD IS different. Maybe it does have deeper bass, and more prominent highs. It's hard to say.