Hello! Thanks for the kind words and for the question! Join me now as I write too many words to answer your concisely-worded question!
My first gut instinct is to tell you to for the Custom One Pro Plus.
That’s the only one out of your list that I can safely guarantee shouldn’t fatigue anyone with their treble. Plus, with the bass port sliders, you can amp up the bass or tone it down to your liking…which to me puts them squarely as the winner in a “most fun” contest as well.
The other three are all tuned in a way that has a love-it-or-hate-it treble push. You might find that it’s right on the edge of what you like, or you might find it to be a bit too much. I’ve never met anyone who didn’t have strong opinions on the typical Beyerdynamic treble, haha.
The smoothest of those three is the 80 ohm DT770, if you’re looking for my second choice. It has a tiny bit more bass than the others which helps even things out a little.
The headphone.com graph comparison tool, in spite of relying on older data, is an invaluable tool in these situations. Here’s a DT770 against your HD 600.
As you can see, you’ll get more emphasis in the bass, a peak in the treble…and a mid range that’s different in its character, but in the same general ballpark. The bass is about even to the Custom One Pro on it’s third slider position, so the COP can get even more bass-y than this. Also, the treble on the COP doesn’t really have that same aggressive peakiness, and will probably be closer in level to what you’ve heard on your 600’s, volume- wise. But with a different texture and quality…since a 16 ohm portable driver was never meant to compete with a 300 ohm studio driver.
The Beyerdynamic house sound comes from the other side of the audio universe compared to the stuff you already own. It’s a bit more aggressive, and in studio work terms, it’s more like a spotlight on areas that might need work, rather than a good smooth presentation of the whole, if that makes sense. This, in spite of the fact that both Sennheiser and Beyerdynamic are trying to shoot for a diffuse field compensation curve. Funny, right?
Congrats on the 550 and the HD 600. Those are both fantastic pairs of headphones and you have good taste! :) And good luck in the search! I hope this helped you out in some way.