The comparison ended up being very similar to my LCD-2 on the WA22 and LF comparison. Odd but for those that are scratching their head, read on.
System is as in my signature and the Peak is currently sporting a Sylvania Bad Boy. I have an JAN 5692 on order as a better match for the HD800 but decided to go ahead anyway.
Throughout the comparison the overriding character of both cans was obvious. Getting the similarities out of the way. Both have great clarity and resolution but are voiced radically different. For one even though they were dB matched the LCD-3 sounds a lot louder.
The HD800 was consistently drier, dynamically flatter with superior sound-staging that had depth, width and unique to the HD800 as far as I'm concerned... height. Not surprisingly and pointed to in just about every HD800 review, its imaging is precise and in a class all to itself. Though resolution is high, compared to the LCD-3 its more primary or mid-range focused.
The LCD-3 on the other hand was consistently the more dynamic, fuller bodied and the more driven of the two. It portrays the entire instrument and emotion better. Which in turn makes it more engaging. Soft background cues were more obvious on the LCD-3 as well. Though both extend to 20Hz the LCD-3 is better at making sure you know it does.
On comfort its a no contest, the HD800 is lighter, floats on your head and just gets out of the way. Going back to what I used to consider a 'comfy' LCD-3 really highlights its vice grip quality.
I was really hoping to have two great cans that were different but met my sonic goals but its not meant to be. Though a Q cable and the 5692 tube might improve what I consider the HD800 weak points I can't imagine it morphing into something else. I'd also add that the often mentioned treble peak on the HD800 wasn't an issue or even noticed to be honest.
Now if I prized sound-stage, speed and that mid to back of the hall feel the HD800 would be a no brainer. But I prefer a full bodied, engaging while resolved presentation so... the LCD-3 stay and the HD800 goes.