Just my 2cents.
I got Objective 2 amp recently and connected LCD-2 rev.1 headphones to DX50. Before that I was listening through Heaven VI (direct or via AMB Mini3 amp). The differences between firmwares were not so obvious till now - O2 + LCD2 makes quite resolving setup.
1.2.2 wins hands down over 1.2.6 and 1.2.7 in my view, I think I was even hearing some distortions on .6 and .7 firmwares, none of them are on .2.