Great bass but not similar at all. Merlin more midbass and 3DD more subbass, 3DD needs a good amp for power.

3DD's sound is not close to HD600. Thought you should try Merlin or Miracle ^^ But I recommend about Earsonics S-EM6 for ya sound signature.

Love its sound so much but the extrude socket really disappointed 

Did Anyone here try both custom and universal versions ? What are the differents between them ?

Well, I used westone before and i'm sure that 3DD has a good seal for me within my modded tips, My friends either ^^ Isolation as good as other IEMs used

I tried once and never want to do something like that again =))   Perhaps I cutted foam not well so it made the sound too bad: all became muffle.

U should try my way: cut its bi-flange and use the under part. My friends also fit easily with my mod tips.

Yesterday, I realized my 3DD didn't have filters like guys who get their 3DDs after me. -1 point for UM lab.    My stock cable has problem of losing sound easily in its right side. Not socket problem, it's cable. -1 point again     .... So I'm got  bad lucks or UM did a bad service for me ?

If u mean dampers, easiest way is using lock part of 3DD's leather case. U can also put them stick together to release each one but be careful, don't let them flip, so hard to put them out.

Unlucky for you. Thought your time for waiting was too long, that very dissapointed service for end-user. I ordered and got my universal 3DD through my local dealer and waiting time was just 5 weeks instead of 8 weeks. 3DD's stock tips bring out the worst soundsign and fitting for me. So I...

U should try another tips and better DAPs. Not only me, many guys here who had 3DD also loved it so much ^^

It's impossible. Its bass driver takes all inside space of recessed socket. 

Finally it arrived ^^       Now waiting its sougnsign after burning in...

The sound with stock tips ? I tried some kinds of tip and stocks have gave the worst sound.

Thanks phntmsmshr. 3DD housing is  thicker than other customs obviously. This is really a deficency.