I have the rev2 HE400s. Compared to the HE500 and SRH1840s, they are laid-back in the upper mids (less of a bite with snares, female vocals etc.) In the mid/upper treble, the HE400 are brighter than the HE500 and slightly brighter than the SRH1840. All headphones are well behaved. I don't hear any nasties in the treble. The HE500 has the smoothest treble presentation, followed by the SRH1840. The HE400 treble can be a bit unrefined, but it's probably the most airy (that upper octave air that we almost sense more than we hear). Despite the uplifted mid/upper treble of the HE400, it always has laid-back feel because of the upper-mid/lower treble depression. The lack of attack of the HE400 (in comparison to the HE500 and SRH1840) does sometimes bother me. The SRH1840 is probably the most "aggressive", if you can even put it that way, in this regard.
The SRH1840 is the most "bass-lite" of the bunch. Do not expect bass power or slam. The SRH1840 does not reproduce the lowest bass registers. Although there is a certain warmth to the SRH1840's bass despite FR graphs. While it's treble is articulate and sweet, the SRH1840 bass and mids are the most muddy, ill-defined, and soft among the three. The HE500 has the smoothest mids out of all of them. The HE400 can sound nasal at times. The SRH1840 can be shouty. The HE400 wins in the bass in terms of control and articulation. The HE500 bass is slighty muddy in comparison to the HE400 bass, but still much better than the SRH1840's bass. Both HE400 and HE500 have similar levels of very good low bass extension. The SRH1840 stages the best, not unexpected since staging is not a ortho strong point.
YMMV.