Thanks for the comments so far, first of all. I've owned many headphones through the years. I have the Sony MDR-V6, and MDR-7506 (consumer and pro versions of the same damn thing). I've owned many Sennheisers...some I loved (HD-400s I owned in the 70s), some I came to be lukewarm about (HD-580). I've also owned Grado SR-60s, and SR-80s.
I came to tire of the overly aggressive "brightly lit" sound of the Sonys and Grados. Great for production and mixing (I know you're not supposed to mix through headphones. It's just that I produce much better mixes when I do!), but for just listening to music, I don't want to be beaten over the head with detail. I just want everything there, in it's place. So I've moved to a preference for a much "warmer" sound than what used to be the norm for me.
By the way...just because you've tried more expensive Sennheisers, don't think that means you have a handle on what the HD-435 sounds like. Sennheiser's "family sound" is turned on it's ear in these (and many of their newer 'phones). Bass is FULL, and very extended. Mids and highs are BACK WHERE THEY BELONG, not up front and 'in your face". If the MRD-7506 and Grado are "solid state" sounding, the HD-435 sounds more "like tubes". I know that's a lame analogy, but there's some truth to it. Though perhaps not as strictly accurate, the 435, to these tired old ears, is quite a bit more "musical".