By design, perhaps - HD600 pads are known to have a relatively shorter service life. Ditto the K701/2, whose pads have the shape that adds toe-in to the drivers (vs the mount on, say, the newer headphones like the HD800 and T1). I'm not about to spend on the HE500 and a proper amp for it just for cheaper pads either.
There's also the possibility that you might not be noticing the effects the on the sound as much as I do, even if the wear on the pads puts their cups at comparative distance from our ears (note that when I stop using HD600 pads it's not like they already look the part, like I've worn them for a week straight and will never go back from that compressed shape). The wear on the HD600 pads makes for sharper treble and mudslide-like bass and bass drum notes. If your music doesn't have such notes, particularly with the bass, then this is hard to notice even on an HD600. Vocal tracks for one unless they have high pitch sections don't sound different.
A better suggestion is find a pair of headphones that don't kill the cups every few months. Seems like a great way to waste money. I have a spare set of HE-500 pleather pads (the velours are too itchy for my tastes) and the ones I'm wearing are virtually identical. I've had them for at least a year and a half so... Yeah. Read that as you will.