I've done a few basic mods to my T50RP, but I haven't dropped it into a new enclosure yet.
The T50 is warmer and more rolled-off at the high end, which means it doesn't have that sparkle that the Beyers do. It's a bit brighter now that I removed the thick fabric on the front of the driver, though.
Some quick impressions:
T50RP:
-the narrow, shallow soundstage is definitely noticeable.
-everything with these headphones sounds remarkably uncoloured
-I really like the bass I get from these now they are dampened better
-really nice midrange--not quite as nice as the T40v1 pull off--but vocals are remarkably clear
-these don't really sound like open or closed headphones.
-I don't find them unmusical, but they are a highly analytical pair of headphones (the ability to capture everything seems more important than a cohesive overall sound)
DT150
-these have a really wide and deep soundstage for closed headphones
-the bass has a bit more impact, but a bit less definition
-as much as I love the sound these create, they do sound like closed headphones
-I really do like bright headphones--those cymbal and high hat hits are awesome with these cans--creates a sense of air and space around instruments
-they are a bit more coloured (compared to the T50RP, at least), which creates a nice cohesive sound. They are easier to listen to, but it takes more concentration to pick out the parts. It's easier to just get lost in the music.
-vocals sound good here, too, but they don't stand out the same way. They become just another part of the mix
-one of my only minor complaints with the DT150 is their tendency to get just a little congested on busy passages
So, yeah... stock, I prefer the sound overall of the DT150, but if I really want to focus on the music and concentrate on how music was put together, the T50RP are a better choice. Ultimately, it depends what's important to you. All bets are off on how these will compare once the T50RPs get the Thunderpants treatment.
Hope that helps.