With ESW9 I'd be scared s***less the whole time due to their fancy finish, fragile cable and construction. My impressions:
SRH840:
+Good isolation, decent imaging and details, robust design, interchangeable cable, rather comfortable, balanced sonic performance, very natural treble (you can distinct the differences in volumes the drummer hits a hi-hat or other cymbal with), easy to drive (iPod compatible).
-Big, very big, heavy/clumsy coiled cable for pocket-portable use, don't stay on your head very well, though refined not very involving or musical sonic presentation (to my ears, need I say).
ESW9
+Very light weight, truly portable dimensionally, comfortable, stay on your head, very involving, musical and a bit upfront presentation though not as refined vs. 840s, displays notes/pitches more "accurately" or with more variation/contrast, better transients, great with jazz and brass instruments in general, also great with progressive fusion/rock, A balance I'd love if the bass was a bit more as in the 840s.
-Seem very fragile, do not isolate unless blasting at high volumes, bass doesn't go as deep and isn't as snappy as in 840s and a bit more prominent in relation to the rest of the audio spectrum, treble not as natural or refined than with 840s.