The clamp lessens with time. Just wear the thing, and within a few weeks it will have loosened up. Fully extending and carefully bending the metal adjusters outward can also help, as can placing the headphone on something slightly wider than your head (e.g. a stack of books) for a day or so. I would suggest you try other remedies before this last one, however, as it will put pressure on the pads, which might wear them out prematurely. They're $50 for a new set, so anything that potentially lessens their lifespan should only be done sparingly and as a last resort.
As for the bass, that's just how the HD 600 is. That's its one weakness; being a 90s-era open back dynamic, it's never going to have the bass extension and grunt you get with closed headphones or planar and electrostatic designs, or even some modern open dynamic headphones. Every headphone has some compromise. For me personally, what the HD 600 does well (overall tonality and smoothness, lack of treble bite) outweighs its shortcomings, but that's always going to be a personal choice. If the missing bass matters more to you than it does some of us here, that's a perfectly valid conclusion.
In my neck of the woods, there are precisely zero places with interesting headphones to try. I've unfortunately had to try everything I've ever bought by ordering it in and living with it for a while. Some of it had to go back, either because it didn't meet my performance expectations or because I couldn't stand wearing it (I'm looking at you, NAD HP50). Despite being relatively picky, I ended up with far too many headphones anyway. Especially since the HD 600 gets used about 90% of the time these days, with the remainder being times when I need isolation or portability so I'm forced to choose something else.
Setting aside everything else, the best thing you can do is just relax and enjoy the music. Worrying about audiophile stuff is a good way to end up listening to gear instead of music, emptying your wallet, and not ever being satisfied. Obviously, if you can't get into the music because the sound just doesn't work for you or the fit remains distracting even after some time, that's a problem, and it's not one you should force yourself to live with. Crate 'em up and try something different. But in the short term, just let the HD 600 work its magic--you've got to work your pair in to loosen up the clamp, anyway, so you might as well do it while listening.
