Hi!

You'd need to give us an idea of what kind of budget you're willing to work with for source and amplification equipment.
That said, I'd look into the Audio-gd NFB10SE, a one-box solution with great horsepower for $500, or the Schiit Lyr tube amp ($449) + Schiit Bifrost DAC ($349 w/o USB, $449 w/ USB). The HD 650 has the benefit of scaling extremely well with equipment uprades, so that's also an element to which you can give some thought.
The budget... well let's say I can spent $1500 - 1800 tops including new headphones.

I may be going against the grain here but I'd say invest your money in a top-of-the-line headphone first and then upgrade your amp/DAC later. You can either do that or go ahead and get mid-tier headphones and amp/dac right now, but when you want to upgrade in the future, you'll have to dish out extra on all 3 components.
I went for the Beyerdynamic T1 while I still had the E7/E9 combo. Honestly, I think it did a pretty good job with it although the dynamics were noticeably compressed (intense passages seem strained). I've since upgraded to a Meier Concerto and MS II and am very happy with my set-up. The improvement in resolution and overall realism of sound is quite noticeable. If I had initially split my money between a mid-tier headphone and mid-tier amp/dac, I don't think I'd be at my current level of contentment.
Btw, what's your preference in sound? Do you favor a lusher and richer tone or a more dispersed and open/airy one?
As for sound preferences, to be honest I like the sound of my HD598. Its “airness” and bass. Since yesterday (btw I live in Europe
) I've read couple of comparisons between HD650 and HD598 and have to say that there are quite a few people that prefer HD598 over HD650?
At the end of the day I just want the sound to be reproduced as close to live experience as it is possible if you know what I mean.
"I may be going against the grain here but I'd say invest your money in a top-of-the-line headphone first and then upgrade your amp/DAC later. You can either do that or go ahead and get mid-tier headphones and amp/dac right now, but when you want to upgrade in the future, you'll have to dish out extra on all 3 components"
Well that’s why I though about buying top to the line Sennheiser at firstJ.
Thank's for all the comments and waitn for more 
Regards
Mike














