Ah, very true.
Perhaps a different wording is in order. Not a more "current day" driver, just a "technically improved" one. Not sure if you've ever compared the PX100 to the PX100-II, but I heard the exact same thing in the sound. The PX100-II had more body, was a bit less grainy, and gave the music more soul. Ever heard of reviewers mention the HD25-II driver is "aging" by the sound of it? I'd say the same thing toward the F1 vs the MA900. The F1 is still awesomely fantastic though (as is the PX100) but they have been, IMO, been outdone by a successor.
And I enjoyed it's tonality over nearly every headphone I've tried. With the special occasion of the HD600 and HD650 sounding a bit better, I still would take the PX100-II due to it being significantly cheaper, and also more comfortable to wear (not afraid to abuse it a little either). As the case of the HD598, I've had that darn headphone three different times, and the PX100-II came out on top as far as tonality and presentation went. And then you have the obvious: price, portability, comfort, etc.