I like the AKG K-240 DF's myself. I believe they are discontinued, but I've been told the ones that replaced them are even better. I've spent years with my 240's and absolutely love them.
While I'll never consider myself a purist, I can say that though my mixes were always fair in cans, they didn't improve until I got a good monitor rig. Though I completely understand what it's like to be forced to mix in cans, I also whole-heartedly feel you are your own worst enemy when you "just accept" that and don't look to better your monitor environment. Though great mixes have come from cans, in my opinion it's a much better situation to listen through good monitors to where you instantly hear what needs to be fixed. In my opinion, no matter what cans you have, you just about always have to compensate for something or make a decision that is really not audible.
Ever make a mix, take it out to your car, write down all the changes you'd like, go back into your studio and not hear these changes in your cans or monitors? So you do what? You compensate for something you don't really hear. You know you have too much bass going on, yet you don't hear too much bass in your studio. So you pull it out anyway and keep on experimenting until you pull the right amount of bass out, or find the right frequency and cut it.
I just wrote an article for Wusik online magazine and one of the first things I mention in it, is not to mix in headphones. If you truly want to become a better engineer and have better, more consistent sounding material, you need good monitors with monitor correction and some room tuning. But if I were in a situation to where it was not an option, the AKG's are the best cans I own that give me mixes that are very close to what I'd come up with on my monitors. They still fall short, but I'd call that "fairly acceptable".
Those Sony MDR's you have jwh, very bass heavy (at least the 5 sets I have here are) and lack mids. They are great cans though because they are loud and drummers love them. I'd never attempt a mix on them though, that's for sure. I really think you'd like the AKG's. They are about $100 more, but well worth it in my opinion. Flat sounding, yet accurate.
-Danny