+3 to the above. Quite a lot! Like Austin and everyone has said, HD650 responds to every little change and upgrade very well. Despite the praising of the resolution of some of the higher end cans, the 650 can easily out-resolve anything I ever intend to throw at it, including the rest of my gear!
I've had my HD650s' 5.5 years or so. Originally used a headroom Micro-Amp and an Onkyo SR-504 as the DAC. It was ok. I eventually discovered I didn't like the Headroom for the HD650s, but that wasn't until much later. It's not just about "quality" of the amp but also the "synergy" of it (mostly in the form of the output impedance matching, plus allowing the subtle colorations of the amp to match well with the headphones.) I'd improved on the sound with an EMU-0404 DAC and the Denon 2310CI AVR as the amp (one of the better AVR headamps out there, it uses the main amps.)
I played with a Cardas (gray) cable for them, and wasn't impressed, I heard no difference.
However, when I plugged the HD650's into the Lyr, even with stock tubes I went "Wow!" the very first note (a plucked guitar string.) I knew the sound of my Hd650's like the back of my hand, they had between 1,000-2,000 hours on them. And the note I heard was unlike anything I'd heard from them before. That doesn't mean it didn't sound like HD650, but the timbre, tone, and physical "dimension" (weight?) of the note was more alive than anything I'd heard from them before. Senn likes tubes...
Then I added Bifrost. Not as big a "wow" moment as the amp change, but it was still a "wow" moment that I noticed immediately (note this is after running several DACs that I did not notice much difference on, namely the AVRs and the 0404.), especially in complex layered passages. There's nothing muddy about the headphones, the problem was the DAC that would jumble busy passages of music. Electronic such as Enigma really fell victim to an inferior DAC since it's always busy.
Then I plugged my Cardas cable back in (I'd switched back to stock for a while.) This time I did notice a difference, subtle but there. In fact I wasn't fond of the Cardas's tone combined with the already warm amp. Then I bought a Silver Dragon, and I really love what it does with the sound. Beware that cables are the absolute last item to consider in your chain. The cost as much as hardware and provide a fraction of the difference. They're the finishing touch to dial in the sound where you want it. But after you've got the rest of your system in good shape, cables, at least for HD650, can make an important bit of final adjustment. Cables are expensive, and they're not magic, but they can make subtle EQ tweaks and other tweaks as the absolute last detail in the setup. In a sensitive system those differences can be heard, but on less sensitive systems, they appear to make no difference.
And finally (which should have been first) I upgraded my Lyr tubes. Another "wow" moment.
As long as you like the basic sound of the HD650 you can build upon it almost infinitely and continue refining it. Where people tend to go wrong is they dislike the basic sound of HD650 and instead of buying different headphones, try to "fix" the thing they dislike about it which for them is the sound signature, not the performance. I have other cans. I like my other cans. But no matter what direction I go, I always seem to come back to HD650 as my favorite.
I mention Lyr since that's what my experience in upgrading was, but I've heard great things about the Crack & Crack+Speedball, Valhalla, WA6, and many others. Go with what appeals to you, but do upgrade. E9 is ok, but you can get a lot more out of those HD650s!
good luck to you!
