I've just received my LCD-2's (single ended) and did a brief comparison with HD650's (balanced) and it is quite amazing to find HD650's keeping up with the LCD-2's which are in a different league altogether. Sure, the LCD's have better top end, speed and superb bass response but each time I wear them, they some how remind my of my HD650's. Don't know if its the mids, the tonality or the balance but it feels like LCD-2 is the bigger brother of HD650's. Although there is one area where HD650's completely demolish the LCD-2 and that is sound stage. LCD-2 has a very closed in presentation compared to HD650's. Its a testament to how good HD650's really are.
Thought I would sell my HD650's after I got the LCD-2 but now I not so sure.
I really don't find the LCD-2s and HD650s (used to own) similar...maybe both are NOT bright headphones...but that's where the similarities ended for me. I was not a big fan of the HD650s. YMMV.