Over in sound science there is a debate about whether or not you can, in theory, EQ 2 completely different headphones to sound the same. Given infinite adjustability and perfect knowledge about a given response curve, I would assume that such a thing is possible.
But to me, it is always better to climb up the quality ladder when possible. It is much easier to tame frequencies than to "boost" them without causing distortion or exceeding a driver's physical limits. If you start from a very good sound, it is a lot easier to get to a "great sound" with some tweaking. Also, if you ever take the phones away from your EQ, say, to your friend's house or some other room, you give up all hard work. Great sound right out of the box is always awesomer. The superior physical limit of one driver over another will tend to provide greater SPL potential and headroom as well, which has a value in some applications.
But given all the other variables - source material, electronics, listening environment / conditions and so on, I am not sure you can really solve all the possible issues with EQ alone - it is just one tool. In a car environment, for example, bass response is improved less through EQ (in my opinion), and more from careful positioning, time alignment, and phase. You can't get a "tight bass" from a massive sub 7 feet away from you and a midbass / tweeter 20 inches from your ear. You just need correction for that that does not come from an EQ.
Also, I am not sure any amount of EQ on a 30-40mm driver can replace the massive diaphrapms in the Planars. Surface area advantage will tend to produce a sound that I don't think is replicated by EQ. Easiest comparison is to think of any system you can with 5-8 inch drivers played at max volumes versus another systems that uses 10-15 inch drivers. Or, better yet, put a 3 inch shelf system next to full size towers. In theory, they could sound the same up to a certain SPL with perfect EQ treatment, but I would put $1,000,000 on the full size towers to provide that sound in 10 rooms of different sizes at a greater overall volume. Plus - you would be more likely to enjoy the physical fun of your hair tingling, hair moving, and ears feeling like they are getting stuffed. Moving more air is moving more air.
Finally, Headphones are an utterly different approach to sound that arguably, renders my previous examples irrelevant. A big problem is that the optimal frequency response of headphones is still mostly unknown - since humans perceive headphone sound differently from cans than from speakers. The "flat response" generally desirable from speakers does not hold for headphones. Genetic variations in hearing and user preferences will also play a role. Have you ever just hated the sound of a particular instrument for example? Sometimes I hate trumpets and other brass instruments, but this sure won't stop the world from making them. Anyway.
