Im a firm believer in burn-in... psychological burn-in. Why fight so hard to maintain an imaginary reference sound that is unverified accurate but just what you're used to. I dont think any of my headphones are perfect and all of them are various forms of compromises anyway. I like each of my headphones for their differences. Many from my age group started with iPod earbuds, that was our reference. We then experience and learn and our perception keeps changing and evolving. Yesterday's Hi-FI is today's MID-FI. Yesterday's holy grail is today's major inconvenience. ...

The most common reference headphone seems to be Sennheiser 6X0 series, if its regarded as neutral then all other headphones will sound wrong. But 6X0 is not perfect nor the best in any single category. I dont enjoy 6X0 series but I understand its very person-to-person dependent based on a bunch of factors:
1. HRTF
2. average SPL
3. upstream source
4. preconceptions
5. preferences / priorities
6. hearing acuity
7. noise floor
8. etc

RS2x (F-pads) is superb from averaged tonal and averaged technical quality, imo 09/29/2023. RS2x (F-pads) is worthy of replacing any one of my headphones (aside from closed-back utility). RS2x is like a super version of one of my all time favorites: Porta Pro. There is quite a price disparity: $550 vs $35, diminishing returns are obvious. Perhaps SR60x (F-pads) or 225x is ultimate value depending on your predilections of course.

Best regards from iPhone dongle,
-Philimon

edit: to add to list, and may edit in future to add specific comparisons
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Update (11/03/2023):
Another F-pad comparison, RS2x vs SR325is/e:

SR325e faired better for most songs when it came to bass quantity. SR325e seemed warmer and sufficiently bassy for electric bass guitar (for example) while RS2x always left me wanting. While both were meh in electronic bass beats: RS2x not enough quantity, SR325e not enough extension.

RS2x wins in terms of microdetail. All instruments were more defined and separated with RS2x with more levels of detail seemingly independent of frequency response. Sometimes SR325e wasnt capable of clearly separating instruments in complex passages while RS2x did fine. And SR325e was less capable in microdynamics, for example missing undulations in instruments including vocalists.

I didnt get the sense that SR325e was bright. I got the sense that 325e was relatively neutral to RS2x, with RS2x being slightly mid-centric. Or in other words, my tonality preferences align more with SR325e. I also listen at very low volumes so a more v-shaped tonality probably works best for me according to Equal Loudness Contour.

RS2x deficiencies in tone are not made up for by its technical prowess. imo I often lack appreciation for technical qualities (outside of bass) and prioritize my perception or preference of tone, so ymmv. Probably I have meh hearing accuity, or my upstream rig is not synergistic, or whatever.

SR325e is like PS500e in that the cables are much quieter. However, the velcro straps on RS2x do work well so not a big deal to me.

In conclusion, I prefer 325e (F-pads) vs RS2x (F-pads).

Already well documented that these are pretty good and nice bang for buck.

I got these used recently. Bought to use as parts, but Inhave to say I like them better than recent Hifiman 5XX and 400se. A little less grainy, more linear frequency response. Still a little sizze up top around 8kHz like Hifiman 4XX, wish they had a little more bass like 4XX.

Con: Too cheap. The price for higher strength magnets is probably just a few dollars. Hifiman charges hundreds or thousands of dollars difference between models. Fang’s phd was in marketing or nano tech? Good work!

edit: some more pics for your viewing pleasure.