Chris - just from my own experience - I had the E7 + E9 combo, and also both the DT880 250ohm Pros and the DT880 600 ohm Premium.
The E7/E9 will get them plenty loud - but IMO unless you're specifically thinking of going OTL Tube - I'd stick to the 250 ohm. The E7/E9 did a much better job with the 250s - than it did with the 600s.
The E9 is outputting 80 mW into 600 ohm, my NFB-12 (which sounds pretty good with the 600's) is 150 mW into 600 ohms. My LD MKIV is somewhere around 500 mW I think. It wasn't until I started comparing that I really noticed how well the 600 ohm scale with more voltage. They are pretty sensitive headphones overall - so straight power doesn't seem to make as much difference as higher output voltage.