Since this is an older thread and i'm sure the poster has found his headphone already I'll suggest the Koss Pro DJ 100.
It has the same sound signature, but maybe a bit less bass, but it is closed. It doesn't sound like a closed headphone though for some reason.
EDIT: Has less bass and the same slightly rolled off highs. The highs feel similar to the HD-600. Not like the HD-650 though I think. The mid-range is probably a bit better then the Porta Pro, especially with vocals. I think this DJ pro is more like a studio monitor, but still fun to listen to. I find them to have a good amount of bass, but some may disagree. A tad less then the SRH-840 maybe,
Basically the Porta Pro has a sound signature I prefer. Many of the headphones in my signature you may like, especially the HD-600.
Edited by tdockweiler - 9/12/10 at 12:19pm