I can't speak to the K371 but the 7510 to me was just a darker and more comfortable version of the 7506. I really don't like the 7506 but I still have a pair. Also any monitoring headphones really aren't going to have any soundstage. They are all designed to be very narrow ... for monitoring. The 900ST is just designed interestingly to where you could mod it to give better soundstage if you wanted to. I am a huge fan of the 900ST hands down. But I maybe bias because its one of the few headphones that is actually made still in Japan. lol
So which closed cans for produicing would you recommend? And which mods improve soundstage for 900st? Sound wise are they clearer or anything?