Well... "that pretty well settles it then" - I'd always thought the HD650s might really clear up, with the right set up in balanced mode... but... it doesn't sound like it. I thought they improved a lot with my DV332, and the various "tweaks" for them (i.e. de-veiling, cable, etc.) - but, they never quite equaled my other phones.
They just aren't ever going to be as clear as a lot of other phones (e.g. AD2000s, RS-1s, D5000s, HD580/600s, etc.) - so, why persist in the pursuit of clarity with them, when we've got so many other really great phones to choose from.
May they "RIP"... HD650s.
I've never modified my HD650's other than taking out the earcup foam which I found made them cold on my ears and a little bright so I put it back in.
I'm not a cable believer (undergrad degree in physics just won't let me believe especially once I did the calculations on the filter transfer function, it's the PhD engineer in me I guess) so the D5000s are stock cables and the HD650s use the headroom "stock" balanced cable (one plus is this comes with a SE-Balanced converter).