I'm playing with an E7 and an E9 now, to see if I'll keep them. In some ways, the E7 is the better performer and will work better with a wider selection of headphones. It has a near zero-ohm output impedance, which means that it can deliver a uniform frequency response to almost any load. The E9, on the other hand, has 10 ohms and 43 ohms for the output impedances off the 1/4" and 1/8" jacks, respectively. I verified this with measurements. Some headphones will not get a flat response with the E9, especially off the 1/8" jack.
Headphones that won't be affected are those with high enough impedance (e.g., HD600), and those whose impedance doesn't change much over frequency (e.g., ATH-M50, LCD-2). Some headphones (like the SR-60, HD598 and many others) will get a slight hump in their mid-bass response, and the effect will be more pronounced when plugged into the 1/8" jack. Some listeners might like this.
Single-driver balanced armature IEMs will sound brighter on the E9 than when driven by a low-impedance source, like the newer iPods and iPhones. The E9's output impedance isn't quite high enough to make an ER-4P sound like an ER-4S, even off the 1/8" jack. But some folks might think they're now hearing more "detail" off their SE310. On multi-driver BA IEMs, the high output impedance could impart an unwanted coloration. On the SE530, for example, the midrange response around 1 kHz is noticeably higher than the output at 5 kHz. Off the 1/8" jack, you'll hear the hole at 5k, now 7 dB lower.
The E7 headphone output is a much stiffer voltage source. I was hoping that the 1/8" jack on the E9 would perform the same way. As it is, either jack on the E9 is unsuitable for some of my IEMs, but if I were forced to use it, I'd use the 1/4" jack. But I guess that's not what the E9 is for. If you get the E9, you'll still want an E7 to power the cans that need an ideal voltage source. If you have the E7 only, you'll probably be OK with almost any load. It's such an inconvenient shape, though, if you really want a desktop amp.
That said, the E9 sounds fine with with my full-size cans. The output is clean; I haven't heard any noticeable noise or distortion yet.
Edit: Typo. Changed 33 ohms to 43 ohms for output impedance off 1/8" jack. Just noticed now--sorry. NWAVGuy just published measurements of this thing, and he got the same figures, and it prompted me to look at this again. This is why it's good to have many different people doing tests.