Your review is excellent: comprehensive, logically presented, and very perceptive re these 2 headphones (Aeolus & HD6XX). I found myself nodding in agreement often.
Perhaps the best feature of the review IMO is your decision to compare these 2 particular headphones, which are about as different as 2 high impedance open-back dynamics can be. I'm lucky to own a JAR650, an HD650 heavily & creatively modified by Jupiter Audio Research to ameliorate the same flaws you mention: bass that resolves less than mids & treble; subtle sense of congestion & veil; overall lower resolution than some other designs. The JAR650 improves these aspects with significantly better, more tuneful bass; improved dynamics; clearly more resolving & dynamic treble, and the same musical midrange.
I've compared the JAR650 at length to the ZMF Aeolus (mine is blackwood). Interestingly, despite the clear improvements in JAR650 vs stock HD650, there remains a great divergence in their sound:
- Greater overall resolution & detail on the Aeolus (this headphone doesn't get nearly enough credit for resolution)
- Obviously better bass, with more depth & impact, on Aeolus
- And more expansive, resolving, dimensional soundstage for Aeolus
Despite its quite effective mods vs the stock HD650, the JAR650 is still somewhat less dynamic, resolving & engaging than the Aeolus. IMO the JAR650 sounds its best used at moderate volume to "monitor" music; headbangers need not apply. It's a clear, engaging window into music, with precise, intimate soundstage and one of the best midranges around. But the Aeolus is simply bigger, stronger, and way more fun. It has seemingly endless headroom/power & scales like mad with music having deep bass, strong rhythm/percussion content and dynamics. Yet the Aeolus also delights on the most intimate & nuanced classical music.
I suspect that years in the future, people will describe the Aeolus in terms that mirror the way we now regard the HD650: a reference design, affordable in its class, standing apart from competing designs when launched and possessing real staying power in the headphone marketplace--a headphone you must hear if you love headphone audio & music in general.