Having done loads more research since my last post, I've narrowed things down to the DT880/600 or the HifiMan HE-400. Right up until last night, I've been dead set on the DT880, loving the idea of buying a customised Manufaktur pair, which would put them in the same kind of price range as the HE-400s. But, having read all the wealth of info on these forums, I'm, gathering that the HE-400s do everything better than the DT880s and are a definite step up in sound quality from them. Then again, some say that the DT880/600s are the end of the line cans for them! I'm getting an opportunity to try a pair of DT770s from a friend, so that might give me an idea of the Beyer sound, but I won't get a chance to try either the DT880s or the HE-400s before I buy, so what should I do?! Any help would be much appreciated.
I've also decided to keep my HD598s now, as there are some things about them that I just couldn't live without! Bearing that in mind, I'd like this next choice of cans to be a great compliment to my Sennheiser's. Hope that info helps inform anyone able and willing to give me any more advice.
Oh, I'm also about to try my first soldering/recabling job on both my out of action Grados (SR80s and SR125s), so I may still keep those afterwards, or just sell them towards this next purchase. I haven't listened to my Grados in a few years, so can't remember exactly what they're like now (except remembering a massive lack low end, yet massive upper detail retreival on the 125s.