I have an in-depth HE-1000 review with comparisons to the LCD-X here: http://www.head-fi.org/t/767883/official-hifiman-he1000-impressions-thread/105#post_11633299
From numerous audition conditions, I have found that the LCD-3F has a nice mid-centric tuning with a touch of darkness that is quite appealing. People also highly rate its "liquid smoothness" though I personally did not find it much different in that particular aspect compared to the LCD-X. The LCD-X has more bass quantity and nicer bass quality (with a more visceral sense of impact and weight). LCD-X tuning also has a sparkling treble (more noticeable peak in the 4-6kHz region) and slightly more recessed upper mids. I personally find the LCD-X more appealingly tuned for my tastes. I personally wouldn't recommend either Audeze headphone as a "main" reference set for critical listening, but they do work well as complimentary headphones, so with your HD800 rig already, either would be a good option depending on personal preference of sound signature tuning. They do fall behind the HD800 in imaging, soundstage, note spacing, and clarity.
I do think the HE-1000 shows clear technical improvements over the LCD-X and LCD-3F. Against the HD800, the HE-1K will be relatively warmer and 'smoother' but not to the extent of the Audeze headphones which display a bit of a subtle blurring effect to the edges of their notes. I do personally find the HE-1k technically better than the HD800 but there are other perspectives on this that I don't find unreasonable. The HE-1K maintains a really high level of imaging, speed, and sound stage that I do personally find surpassing the HD800 from audition conditions though the increase sub-bass presence of the HE-1K may give the HD800 the edge in imaging for certain source files where the increased resolution of the HE-1k for bass details can mask some other depth perception cues. Sound stage the HD800 may be a tad wider L-R but extremely close to call and I consider them equivalent in that area while the HE-1k definitely has a better sense of depth and height.
I personally prefer the HE-1000 over the LCD-3F. Also prefer the LCD-X over the LCD-3F. Would prefer the HD800 or HE-1K over both Audeze options if only going for one main headphone. For a complimentary pairing the HD800 + LCD-X works quite well though the LCD-X is not as good a technical performer as the HD800 and will likely be designed as a 'fun' pair of headphones. The HE-1k captures the excitement of the bass impact that the HD800 without being too overtly colored while maintaining an extremely high technical performance and that would be the headphone I personally would recommend if your budget can stretch that far. Other considerations is that all Audeze LCD-series headphones are extremely heavy if you are coming from a HD800 background (an extremely light dynamic headphone). Will likely need to do a comfort mod. The HE-1K is much more comfortable. I would say the Audeze headphones have a more premium sense to their build quality using heavier parts. Price point favors the Audeze headphones but I do personally consider them overpriced for their technical performance, so I actually think the HE-1k is the better value despite its much higher price point.
Note these are only my personal thoughts. other people will have different perspectives & ymmv! hope this was helpful
. You did the right thing by the forum's rules. The HE-1000 is probably more versatile, as the LCD-3 is a little dark. The LCD-X would probably be a better all-arounder than the 3.
