I figure everyone who's done even minimal comparison would agree that any released firmware in the 1.2.x series sounds better than anything with a fruit logo and a name that starts with a lower case "i". There may be some changes along the way from v1.2.2 through 1.2.7. These changes are at most about 1.0 dB in the low end. An adult human being with average hearing probably won't notice the difference through casual listening assuming these differences actually exist. I put them all side by side as best as I could and I believe that I noticed the bass increase but that could simply be my own confirmation bias. Several Head-Fi'ers have posted frequency response graphs showing very flat response with v1.2.7 which supports my suspicions of confirmation bias.
My recommendation: install v1.2.7 if you haven't already, enjoy the music, and don't worry about sound signatures.
No disrespect, but I do not agree with this. There is no comparison between the sound sig of 1.2.6, and 1.2.7. 1.2.6 wins hands down! On both my Westone 4cs and my re400s. The depth and air and width is completely different. I was listening to the DX50 with my re400s on the subway today in NYC and was blown away at how good tunes sound on it with 1.2.6. Ihope the new 1..2.8 can bring back the sound quality of 1.2.6 with the new fixed features!