Also comfort is important
You just need to loosen the clamp on these Sennheisers when you get them. Extend the metal arms outward and bend in the opposite direction. They're shaped that way because of how much bigger the box will be if it was sized for a normal head without that much clamping force out of the box.
currently have a dac /amp call Asus called thunder fx which supposed to handle up to 300 ohm of headphone
Just note that with soundcards manufacturers stating "capable of up to xxxohm headphones" it's more like if BMW said the 530i can get you past the speed limit, without due consideration to how it isn't going to get past slow trucks without slamming into oncoming cars the way the M5 would (much less have enough fun pulling out of corners). As for that particular sound card, keeping with the BMW 5-series example, it would be more along the lines of the 540i or the 530D turbodiesel.
Also, more in line with power to weight ratio in a car, that marketing bull ignores sensitivity, so in some cases you can have a lower sensitivity headphone with lower impedance that might not go as loud as the 300ohm headphones with higher sensitivity getting less power at that impedance level.
With these in mind should I really go for the 6xx since the drop would be ending soon and they seem to be a "limited " edition headphone as well. Would I notice a lack of bass in comparing to the Sony?
The Sennheisers don't have any isolation at all. I only use my HD600 to play on the desktop but if I'm listening to music I'd move to my reference system with a DAC-HPamp with a fanless music server (my smartphone). When I get to loading screens even with some background music, if I've been playing long enough that my computer's fans are ramping up, he noise is really audible. Even with my PC running at idle with no music playing the whooosh of moving air is audible.
As much as you can crank up the music, the elevated noise floor in the room will affect what you hear overall, most commonly, it can mask a lot of the bass. On top of that, a closed headphone like the MDR1 doesn't just block out noise - the sound from it particularly the bass bounces around in there. The only way the HD6XX will have more bass than the MDR1 is if high output impedance on whatever the Sony is plugged into is enough of a problem that it EQs the sound, but if you're getting enough bass out of the MDR1 now chances are this isn't a problem.
