Good post.  I can add some anecdotal (read: worthless) data to it.  I never measured my LCD2s but I am fairly certain they had poorly matched drivers.  The soundstage always sounded like it was rotated in left of center.  No software crossfeed would fix it. 

Software (or hardware) balance controls might fix it. However, IMO a ~$1k headphone with severe driver mismatching issues is a defective product.

Indeed, then you have a dedicated equalizer per channel nightmare. If the drivers on a single headphone are off, one can only wonder about the driver differences between it and another same model headphone...Life is like a box of chocolates

I've tested a bit on driver mismatches, here's what I found:
- if it's just volume, then obviously standard balance control/channel gain works very well - I had only one such case though
- if there are major differences in some parts of frequency response, all-pass alone (bandlimited time delay) works better than IIR equalization.
- minimum phase FIR is equivalent to standard IIR in this matter, linear phase... depends, sometimes it's better, sometimes it's worse. Might be dependent on the CSD, but I don't have reliable enough setup to measure this.
- apparently ringing is more important than FR for perception of balance
- a combination of balance plus the all-passes works extremely well of course
- the choice of channel balancing technique is unimportant if the difference is less than 3 dB. At least to my ears. At larger differences differing balance technique becomes important.
- applying crossfeed after EQ does not change the choice of the method - the spatial difference is actually less audible.
- IEMs are more resolving when it comes to channel imbalance, fortunately most IEMs are well balanced.
- narrow peaks of imbalance are much less audible, but they affect subjective "locatedness" of the sound - make the soundstage seem fuzzy and less defined - especially if they are numerous
- highs imbalance affects positioning much less, but messes with the depth of the soundstage
- bass imbalance affects positioning the most, but...
- mids imbalance is the most audible most of the time
- equalization is more accurate when done separately on each channel than centering a mono source - slightly better results, mostly in "locatedness".
- correcting resonances based on plain CSD results in wrong, weird sound. Manually correcting based on *tweaking* the phase response using the CSD works very well, but is very time consuming.

Caveats:
- I can hear misbalances down to 1.0 dB, straight dips and peaks down to 0.75 dB directly, 0.5 dB with many tries.
- The imbalances in IEMs were confirmed using a 5mm x 65mm silicone tube coupler with a silicone "horn mouth" fed into a measurement mic. (volume matches my ear canal volume - ~1.2cc; also checked with 1cc and 0.75cc.)
- No such confirmation for headphones yet. My current ghetto coupler (with silicone foam) is too resonant on its own, giving invalid CSDs and not quite valid THD. Needs more mass and back surface less reflective than plywood.
- Correction was applied in three ways: rebalancing only using mono source; mono equalization plus rebalancing; stereo equalization
- All filtering done using a Python reimplementation of algorithms ccirls (FIR) and mpiir_l2 (IIR) in this thesis: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.6.9336
Verified against CAPS equalizer and Electri-Q - these algorithms provide audibly better results. FIR one is not great for minimum phase though.
Minimum phase FIR was achieved with better results by applying mixed phase (33%) CCIRLS with windowed cepstrum method to convert to true minimum phase.
- CSD correction was attempted only on one pair of AKG K271. Very time consuming, took 2h to get a good filter.
- Program used for these corrections will be released soon.

These results were derived using:
- 3x AKG K271 headphones - notable ringing, some dB imbalance in bass and subbass; ringing differs between channels and pairs
- 5x Superlux HD681, unmodded - also ringing, general very audible imbalance in 2 out of 5; many narrow imbalance blips.
- modded Hifiman HE-500; HE-6 - no real ringing - well balanced - some narrow blips
- Paradox - some imbalance around 2 kHz, otherwise very even.
- Spiral Ear SE-5 - very imbalanced area in highs, rest is ok.
- Hifiman RE-ZERO, 2x RE-600, 2x RE-400 - quite well balanced out of the box, all have long "flat" reverb ringing; wide shallow imbalances
- Hifiman RE-300a - some mids imbalance, no ringing
- modded Sony MH1 - highs ringing, noticeable midbass imbalance and difference in amount of air for each channel
- One "dark chocolate" Beyerdynamic DT-1350 - "wavy" balance everywhere, but good general balance

Couldn't test with these due to no imbalance being detectable at all. Great QC is great! 
- VSonic GR07, GR07 mkII, 2x GR06
- Brainwavz M2 (this sounds like VSonic GR06, same OEM?)
- Brainwavz B2 (well matched TWFKs)
- Sennheiser HD600, 2 pairs, old driver
- Sennheiser HD650, new driver
- Hifiman RE-272 - one narrowish imbalance at 1 dB/2.5kHz is all I think this has, I couldn't ABX the difference.
- Beyerdynamic DT-770 - new, wide blip in highs, 1.5 dB, but couldn't ABX the difference.

Couldn't fully test these due to only tiny loan:
- Audeze LCD-2 - balance was very good
- Beyerdynamic T-70 - excellent balance, but very uneven FR

Couldn't fully test these, because I couldn't stand the sound:
- everything Ultrasone (680, 840, Platinum)