Seems like your impressions of the K612 are made up of a bunch of different AKG headphone impressions. The K612 mids may be a bit more forward than average, but they still have the very common dip below line-level dB, and midbass rises above line-level and doesn't sharply drop off the dB as bass extends into the sub region – overall the K612 has the smooth shallow v-shape common in high-fi headphones. The older K
701, K702, and
Q701 headphones had bumps in the headband that were pretty firm and caused some people discomfort (me too, in longer listening sessions), BUT the K612 has no bumps to it's headband, so it should shape to your head and feel more comfortable than most headphones.
From what I've read and my AKG experience, I'd recommend the K612 over the HD598. While they both have a decent Frequency Range balance, the K612 is more "fun" and energetic while the HD598 is more "laid-back," perhaps feeling oddly "removed" from the energy of Metal. The K612 wins in comfort unless you like more clamp, the HD598 headband is also more rigid and hard. Some people really love the Sennheiser experience, they do make great headphones, but I would recommend AKG.
I bet you're like me, you research things for a looong time before you settle on a choice, but once you make that choice you are very satisfied with it. I waited a few years to get my first AKG, and Let's not talk about how long I waited to get my first car that wasn't a hand me down... But take it from experience and do yourself a favor, wait a few weeks to make sure a desire isn't an impulse, but if you take too long and research turns into stalling tactics, you'll make your head spin. Eventually you have to be decisive, and you'll be glad you did