...I'm going to clear up some misnomers about the E7/E9. Firstly, this combination *can* drive the HD650 with room to spare. The E9 puts 80 milliwatts into 600 ohms; that is a TON of power. You can blow the 650s off someone's head easily with the E7/E9.
Now, other amplifiers can drive the 650s "better" in the sense that they may increase the soundstage, presentation, extension in the lows/highs more substantially, etc, but that doesn't mean that the E9 *can't* drive the 650s; it just means that other, more expensive amps do it better (huge surprise).
As someone who owns the E7/E9, the 650s, and is a former owner of the HD595, I STRONGLY urge you to return your 598s and go for the 650s. If they are in the same tier as the 595s, you will hear a difference. The 650 was Sennheiser's flagship headphone for quite some time; if you choose not to upgrade, you will get upgrade fever and want them soon enough. Better to just bite the bullet now.
There is a nice synergy with the E7/E9/HD650 in my opinion. Also worth noting: if you like the sound signature of the 598s (more distinct treble, brighter, etc) then you may actually prefer the HD600 (there's nothing wrong with that). It is a more detailed, analytical phone. The 650 sounds like a dark jazz club, has less in the treble (some say veiled), and is overall a richer headphone. With either one, you won't go wrong.
Hope this helps!