OK I have some impressions. Please see my profile for the full list of equipment used.
The contenders are LCD-3F ($1950 new, ~$1500 used), HD650 ($300 new if you wait for a sale). HD650 was driven single ended from Ragnarok and is brand new with approximately 50 hours of burn in. Due to amp topology a balanced cable will improve it but I am highly doubtful my impressions would shift dramatically. LCD-3F was driven balanced from Ragnarok and was manufactured Nov 2014.
I prefer a slightly dark signature as I find myself sensitive to harsh mids or treble. I also primarily listen to latin music which contains a ton of mid and treble energy from brass and percussion instruments and becomes fatiguing (almost painful) VERY fast if there are issues in the upper mids/treble.
Overall FR curve as I hear it: (I have not measured either headphone; I should request the FR chart from Audeze, will update when I do)
HD650: Neutral mids/treble, minor midbass hump
LCD-3F: Neutral bass, recessed mids, slightly rolled treble
Quality of Bass:
LCD-3F > HD650
The LCD-3F is clean and punchy like good bass should be. Only the physical sensation is missing but you cant get that from a headphone. I feel the LCD3 does a very good job here. The HD650 is a bit flabby and uncontrolled. It's not bad but obviously inferior to the LCD3.
Quality of Mids: HD650 = LCD-3F
I feel they are about equal. The LCD3 is recessed however I don't hear any issues and the resolution/plankton is very good for both headphones. The HD650 is more appropriate for critical listening however the more forgiving/colored nature of the LCD-3F makes it a more enjoyable listen for poor masters.
Quality of Treble: HD650 > LCD-3F
No contest, HD650 wins. I hear what I think is a minor hotspot somewhere around 8K on the HD650 but it does not grate and overall the HD650 is still significantly superior to the rolled LCD-3F in the treble region.
Tonality:
HD650 > LCD-3F
Small victory for the HD650. LCD-3F sounds a bit "electronic" or "robotic", HD650 sounds quite natural.
Soundstage and Imaging:
HD650 > LCD-3F
HD650 has wider soundstage and slightly better imaging. Unsurprisingly they both get absolutely destroyed by my studio monitors.
Overall:
I am going to wait a few months before coming to a final conclusion but my initial impression is that with my equipment the HD650 is the better headphone overall (and for what I want to do which is mastering type work) although the LCD-3F is better with bass heavy genres and with poor masters. I may end up selling the LCD-3F but it would only because I want to fund other purchases (like a 3D printed tonearm for my turntable).
