I have never heard the 600/650 headphones. Recently got on a headphone obsession and have tried numerous models lately. Some of these, like the highly touted k371 and Dt770 80ohm, I didn't like at all.
Well the DT770 has pretty horrid tonality so no surprises there. The K371 though is a different story, and I would indeed suggest you're an outlier if you didn't find it to sound at least pretty good.
Also, where is "good" being defined for you? You said you got onto a headphone obsession only recently so you may be accustomed to how music sounds on bass-focused consumer headphones or some truly wonky tuning. Music is widely mixed and mastered on headphones like the K371, HD580 and HD600. These tonalities are objectively excellent as defined by wide-scale preference research over a couple of decades, so you may have some acclimation ahead of you.
I was also disappointed in the Harman response when I began my audio journey....where was the midbass hump? Why was there less energy and raw excitement?
However after years of experience with many different headphones it became clear that it was the definition of great tonality. Everything I wanted...rumbling subbass, slamming midbass, realistic midrange, non-fatiguing but articulate treble...could only be found at its best in headphones that respected this target.
You can find
more articulate treble,
more slamming bass and
more energy elsewhere, but there will be sacrifices. That energy will be at the cost of fatiguing highs, and that slamming bass will override the mix. Harman tuned headphones represent the best possible balance.