I've owned DT880/2003, HD650 w/either the Cardas or RAL cable upgrades, and the K701. Right now, I'm a/b-ing the Proline 750 v. HD650/RAL and K701. To be honest, I prefer the 750 to either the HD650 or K701. The HD650 are my least favorite of the three. (Yes, I prefer the 750 w/stock cable even to the HD650 w/upgraded cable!)
My last comparison, in all fairness, should be Proline v. DT880/2005, but I'm almost postive what the outcome would be at this point. The DT880/2003 just didn't quite deliver the critical midrange goods, IMHO.
I am expecting the RAL cable upgrade for the 750 very soon, which should further distance the Proline 750 from the rest of the field.
For me, the Proline deliver an honest, almost raw, midrange that seems as though it's hardwired to my emotional circuitry; together with driving, foundational bass and reasonably extended and balanced highs. While the Prolines may not do any one thing exceptionally, technically well, they manage to get everything reasonably well and balanced. Somehow, for me, they also manage to be the most emotionally involving cans I've heard to date.
(That reminds me: it's to swap out the K701 for the Proline 750.) : )
PatABurd