Why a portable amp? The D2000 and HD600/650 are not very portable. They aren't headphones that you are going to go walking around with. Even if you are using a portable as a source I can't really see those headphones being used as portable. Would a smaller desktop amp that is transportable work? A desktop will drive the Sennheisers better. I use an AV123 x-head with my D2000 and HD600. It's a $200 amp and is small enough and contained enough to be transportable from one place to another. I'm happy with the amp. It drives the HD600 and D2000 with power to spare. More expensive amps do do better, but the x-head gets you well into the playing field. I'd rather have the x-head than any $300 or less portable amp (based on the portable amps that I've heard which admittedly isn't a lot) especially if you have a HD600/650 in your future.
The D2000 is my choice for electronic and most rock and most metal. There is some 80s era hard rock or heavy metal that doesn't work so well with the D2000. The midrange vocals get sucked out due to the way the songs were mixed or EQed or something (I'm not sure what). Whatever it is, the problem has been limited to just certain songs of that style from that era (some Sammy Hagar, Scorpions, and the like). When that happens I switch to my HD600 and all is good. Otherwise the D2000 has been generally good for rock. I have "Lightbulb Sun" by Porcupine Tree and the D2000 does well, but the sibilance in the vocals can be a little more than some would like. I'd rather listen to "Lightbulb Sun" with the D2000 than the HD600, but either would do.
Jazz on the D2000 is a little hit and miss. Heavy Weather and anything with Jaco is wonderful. However, I have noticed that some jazz trumpet and some jazz sax can be a little too brassy. As if a brassy overtone is getting accentuated. So for jazz trumpet and jazz sax I tend to grab my HD600 over the D2000. I need to get into a jazz mood again and remember to write down what songs I notice the too brassy sound in. When I have noticed the too brassy sound I switched to my HD600 and the sound was good, I switched to my M-Audio monitors and the sound was good, switched back to the D2000 and it was "wrong".
My D2000 is unmodded. Cable mods and similar mods I think are silly unless you happen to have a budget that allows for that kind of playroom. I plan on doing the Dynamat mod to my D2000. I already have the Dynamat and the mod is inexpensive. I may also get the JMoney earpads. I just have to get the motivation to actually do the mod. I have no plans for cable mods. I'd rather put money towards another set of headphones.
I'd suggest getting the D2000 since it has the most bang for most of what you listen to (except for some of the jazz) and then plan on getting the HD580/600/650 later. And if you can manage with a smallish desktop amp the x-head will do ya well and is only $200. That's my setup (x-head, D2000, HD600) and I'm quite happy with it.