I don't "think" HD800 has bass - its about as linear as the MM500, with only about 3dB difference all the way down at 20hz (both are off by ~8db compared to fx harman), and about equal all the way above that, though with the benefit of HD800 having dynamic punch/bass impact, which most planars consistently lack compared to dynamic drivers - but they gain control instead. Until 1k where the MM500 has an weird/unnatural elevation relative to normal targets, causing its presence there, giving an impression of detail, similar to some abyss/focal headphones. Frollowed by an overly present midrange. And then most of the treble is just rolled off, causing the relarive midrange glare or honk. Where as the HD800 has the opposite issue, of overshooting targets slightly in this region.
For no EQ mm500 definitely isn't a winner - arguably neither are. For sub bass its got similar extension raw, but lacks impact, and if calibrated to a target, its effectively rolled off just as the HD800 and most open backs tend to be.
Main difference between the two stems from one having rolled off treble and 1khz/3khz honk, and the other having extra treble energy causing some potential glare. Both have similarly linear bass extension - and are similarly weak in bass compared to preference targets.
You personally preferring the MM500 personally is totally fair, but saying it destroys the HD800, with how many flaws the MM500 has in tuning, is a bit wild to me, and rather misleading.
The reason HD800 is still relevant, like it or not, is because new flagships like MM500/LCD-5 and others remain just as flawed, and are simply different flavours - until someone actually nails a proper FR that doesn't have just as many issues, it'll likely keep being relevant, depending on which compromises one prefers and personal taste.
Anyway, agree to disagree on semantics I guess.