I totally get what you mean, friend, and I totally agree. Although I owned the HD 598 SE for a year, I relate to what you say in regards to my DT 990 Pro 250 ohm, T90 Chrome, and DT880 Edition 600 ohm. I purchased them in that order. Being used (but not amused) to my Audio-Technica M70x, I was impressed by the DT 990's soundstage and details, but irritated by their sharpness and lack of mids compared to the full, realistic-sounding (but duller) HD 598 SE.
That was the first time I started getting suspicious of the headphone world's pecking order. So I got the T90 Chrome out of curiosity to see if there was a noticeable jump in sound quality. Tesla drivers and all, they are refined DT 990s. No poetry needed. There is a noticeable difference in the sound presentation, which I like more, but it's not in the sense of better or worse. The contrast is just in the aspect of "to sound this different, more money needed to be spent by the manufacturer and by me", not in "these T90s have better sound quality because they're higher in the model lineup".
That higher model number and higher cost got me not superiority, but something that fit better the mold of my preferences. Once we reach a certain threshold, it's difficult to hear the difference in sound quality, which is in my opinion objective to a certain extent, but subjective for the most part. There is a wall that exceeds our ear and brain's capabilities, and (fortunately) the price is less than $1000 in my experience. Over that wall, that's where audiophiles climb and start imagining things, getting poetic, and they start using esoteric terms and otherworldly logic.
So after realizing that I liked the T90 more not because they sounded better than the DT 990, but because their sound signature, although still exceedingly and unnaturally sharp to my ears, was more tolerable, I decided to "downgrade" and got the DT 880. Instantly, I liked them more than the T90. I was like, "THESE are the better, superior-sounding headphones if there was such a thing. Man, ah, this is so much more tolerable and doesn't sound technically worse than the T90, just different. These hit the spot. I wanna hug them."
So yeah, I was going to eventually get the HD 6XX out of curiosity, but after reading you and seeing that I'm on your same wavelength, I'll probably skip them if they're not better than the HD 598, but different. I gifted my HD 598 SE to a friend because I wanted more highs, even though everything else sounded acoustic and real. All my three Beyers have a metallic, bright sound signature, and way too much highs and not enough of the muddy mid-lows and mids. In my opinion, a little muddyness from the mids down to the mid-lows is OK, because that's how real-life sounds.
You know what combination I'd like? The T90's airiness and space, the DT 990's lows, the DT 880's highs, and the HD 598's muddy mid-lows and mids...but keeping the smooth, realistic tonality of the Sennheisers (versus the bright, metallic tonality of the Beyers). Those would be my ideal fun headphones.
I'm not an audiophile in the sense of the psychological disease that attaches the big ego of people to their bigger wallets in search of a non-existent audio nirvana, but I do care about sound more than what is deemed normal. I consider myself to have higher polished ears than the average person, because I've been involved in sound producing and engineering for 16 years. It's a fact that working with EQs and compressors and other sound shaping tools polishes the ear and trains the brain to notice subtleties and nuances, which translates in music enjoyment to noticing the difference in sound textures between different headphones.
It find it highly rewarding when headphones let me hear deeper into the music, and if not deeper, then differently. But I
keep it real, because the closest thing to the audio nirvana that audiophiles seek is attained during the mixing stage, not during music consumption with obscenely expensive gear. In that mixing stage the overall sound is still manipulable because every single track and sound shaping tool is still available to be freely adjusted. Then the final stage is mastering, which is when the entire music track is "fairy dusted", nuanced, and receives its final compression and limiting to get the final overall volume and dynamic character.
Once the mastering is completed, that's the music tracks' birth. So audiophiles want a perfect baby, when the baby was already formed by the mixing and mastering engineer. Knowing all this, I buy headphones being conscious that there is a point where there won't be more to be had, and again, it can be reached below the $1000 mark in my experience.