That's interesting. Quite a few people say the bass is muddy, IIRC, and uncontrolled with the NC off. With a good amp it tightens up. Someone else with the DG and NS1000's didn't think the DG's amp drove them that well with the NC off. Hmm, might be down to the individual's tastes, as to how good they sound with the NC off and no amplifier.
It would seem the NS1000's like 300 ohm amplifiers best, which the E5 is, and the Essence amp, even though they are rated at 100 ohm with the NC off. Makes some sense I guess, given that they are 300 ohm with the NC on. Unless of course, using a 300 ohm amp, brings the sound closer to having the NC on, which some people want or are looking for. With the NC off and no amp, although they work fine, it's a different sound, which is to some people's liking, not to others.
Edit: I tell a lie, the Essence amp is 600 ohm.