For the XC to really shine, there are two (maybe three) things:
1. custom foam or material inserts for earpads interior for better comfort and warmer sound
2. very good quality custom cable instead of the crappy stock
3. 99% seal mod (aka Spacer Mod).
I've been experimenting acoustically with my XC since two years already and this is probably the best thing I came up with. LCD-XC are VERY sensitive for sealing and the point is to get the sweet spot sealing. With it, there is no problem with sound isolation or sound leakage, yet the sound is much more realistic with better bass, better imaging and soundstage, without "flat" highs with tendency to become overly bright.
Such things I've got when my earcups on XC were screwed too tight (and there is a risk of damaging the earcups).
But at the same time the earcups can't be too loose (resonances and uneven seal/sound) or with greater distance (hollow sounding, very poor control over bass region, worsened sound isolation).
The mod is very simple and contains 8 pieces of 5 mm x 1 mm x 1mm adhesive rubber pads put in the places where the screws are.
Blue markings are the supposed pads.
The absolute requirement is a 1 mm height. With such, I was able to make them sound the right way. The Mod was performed on other XC and by other people - the same results and very positive effects. As i said, I've had XC with very tight seal which causes them to sound flat with very weak bass and unattractive tone. I dare to say, in my case they advanced with few classes up.
Just give it a try, even out of curiosity.
