Quick PS, just compared sound of the same piece from the cassette deck to a version on the computer. Deck definitely has the advantage across the board. Though, the cassette being listened to was also recorded directly from a record, and the phones were connected via a dual 3.5mm to 6.35mm into the deck.
Edit to PS: Posted the original PS just as you posted Mak. I will look into these suggestions and keep everyone updated.
I'd say definitely an amp problem. If possible go to a store and try them out on a powerful warmish sounding amp and be amazed.
If I plug them straight into macbook, they can get ear bleedingly loud. But the sound is just terrible, no staging, imaging, dynamics, distorted nasty sound. I'm not going to even talk about awful bass performance. Although I wouldn't say they sound muffled, bright is the word I'd use with my experience. Indeed, even cheap 100$ headphones sound better straight from laptop. Don't believe people (likes from ASR forums) who say that if you get volume, all is good. Simply just no. Go to a hifi store and confirm it by yourself. Why is it like that? I don't know, although there's some talk about amps being able to provide enough current and not only voltage.
As far as burn-in goes. Every dynamic (moving) object will wear over time and become softer. And is definitely noticeable with speaker drivers. Speaker drivers will sound smoother over time. It's a gradual process taking place until end of the speaker. Guitar players go crazy for vintage speakers for that reason, even though new reissues are manufactured exactly the same way.
Also I'd highly recommend looking into EQ as you can make headphones perfect for you. Especially big planars as they are very EQ friendly compared to dynamic drivers.
Windows: Equalizer APO
Mac: eqmac or soundsource.
I wouldn't recommend sticking with oratory1990 presets (or any other) or trying hard to adhere to harman target. It's a one-size-fits all target that most likely won't be your ideal. We are all special little snowflakes with distinct ears and other anatomy. And besides that, I use different EQ settings with different amps. Definitely try those oratory presets out though!
If you want something that sounds the best to you, you must learn to EQ. It's a topic that can go very very deep. Mastering engineers pretty much devote their life into mastering this skill. But don't let that distract you. Start off easy by adding a bass shelf and play around with the amplitude (gain) and frequency (where it starts). Gain will determine the amount of bass you want to add. Frequency for bass shelf will mostly determine the warmth of the headphone. At 100hz headphones will sound thin, at 150hz will sound overly thick and bloated. Your ideal is somewhere in there.
Everything in EQ is relative to other parts of the EQ. For example you can make headphones sound overly thick with bass shelf that extends deep towards mids. And compensate with a boost in the upper mids (around 2kHz range). This way you can have body and clarity for vocals.
All these changes are personal preferences and can be very rewarding when you find your ideals. And can be quite fun if you get into it. That said, you still need a good amp, especially if getting into EQ. I'll leave a nice cheat sheet down below. It isn't quite as simple as that though, but will give you a direction.
Also a sidenote on gear: Amps and DACs do make quite big differences in sound. IC amp sounds different form Solid State and Tubes sound different from those. Mostly this order goes from analytical to musical, there are trade-offs picking one over other. And there are exceptions. Some SS amps sound tubey, some tubes sound very SS like. Best for you is to try different things and find what you like. Don't get drawn into SINAD and other measurements, trust your ears over everything else!
For DACs there are mostly 2 categories: sigma-delta and R2R (also multibit goes here). Sigma-delta is newer detailed, punchy and analytical sounding. R2R is more musical sounding one, more natural timbre etc. but you will be trading off details and punchiness of sigma-delta.
I think that's quite a lot for you to get started and hopefully answered some questions. Cheers!