I was presuming you were starting with DSD, i.e. don’t do any processing after the signal is DSD.
Yep if they are converting to DSD, they shouldn’t be doing any processing after that, but (and I know nothing about Audirvana and very little about the Mac environment) there may be a chance that the OS is doing some volume adjustment between Audirvana and the output. Perhaps Audirvana guards against that.
Don’t do a final 6dB gain unless 6dB has been lost somewhere else. At most (if it’s an option) use a 3dB gain at the end. With no final gain you should have the same level of output as input when converting to DSD. DSD has some head room that PCM doesn’t but since you’re coming from PCM you won’t be worse off to not scale the output. Using extra gain at the end would make it really hard to judge the sound between using DSD and not, since then they wouldn’t be at the same levels and louder almost always sounds better.
Yep, it’s always worth a try - no matter which may be the best technically, there are always some that prefer one and others that prefer another. The only way to know where you are is to try it.