I've never listed to an HD280, but I can speak to the K702 on a couple different systems. I don't yet have a good amp for my K702, but they sound noticeably better on my cheap amp (Presonus hp4) and my Yamaha receiver than the K240S and K240DF which I replaced with the new 702. I only had to listen to one piece, classical, and it was Out You Go with the DF, which is a classic professional studio phone. The highs were too shrill. The 240S, which price-wise is about in the same range as the HD280, is also not on the same quality level as the K702 so I sold that one as well.
Then yesterday, in response to another thread, I tried the K702 on my old Dell laptop which has only an onboard audio chip, the Sigmatel STAC 9750, apparently a DAC. Judging from what I've read at head-fi about the difficulty of amping the 702 I expected not to hear anything at all, but it sounded respectable and easily loud enough, even the bass. I have an ultra-low-distortion DIY desktop amp in progress and am anxious to compare the good amp with the unamped phone. On the laptop I also tried my low-end Koss i50 headset which wasn't created for serious music listening, and it didn't sound too bad, though clearly not like a pro phone.
The high-end AKGs, both K702 and especially the retired K240DF, were engineered to be more neutral than a consumer phone for precisely the reason that you note. My use is mainly orchestration monitoring so like you I don't want the phone to alter the sound. The Senn HD800 also sounded neutral to me when I compared at a couple different head-fi meets, an excellent headphone, but it's not in your budget.
Regarding comfort, I have a significant complaint about the K702, namely the bumps on the underside of the headband which hurt my scalp. I rigged up a sock wrapped around a foam strip which solves the comfort problem but is an obvious kludge. AKG corrected the prob with the newer version of the K701 but curiously not the K702. Search on "K702 headband" and you'll see gripes from others, and a pic or two of my kludge solution.