I have owned both, and wound up keeping the 8001.
To my ears and in my system, the 8001 has a blacker background, more low level resolution, more space between instruments, and slightly more depth.
The 840c is a touch smoother, and has a little more warmth in the lower mids, however.
These differences overall were very subtle, and not night and day by any stretch of the imagination.
This conclusion was came to after extensive head-to-head comparisons both in my home rig, as well as on my reference headphone system as well.
To my ears and in my system, the 8001 has a blacker background, more low level resolution, more space between instruments, and slightly more depth.
The 840c is a touch smoother, and has a little more warmth in the lower mids, however.
These differences overall were very subtle, and not night and day by any stretch of the imagination.
This conclusion was came to after extensive head-to-head comparisons both in my home rig, as well as on my reference headphone system as well.