I just received the new "revised" version of the Muses edition and I would have to completely agree with you. E1 Muses is far from from bright. It sounds very smooth and lush.
However, my previous Muses edition was on the bright side and the bass was kind of thin too. I don't know if I received a defective unit or what, but that's what it sounded like. I was thinking it is possible I might have received the Plus edition wrapped in a Muses box? I never really did open it up to confirm the op-amps.
But whatever issues I had with my old Muses have been resolved. The new Muses I received directly from ASUS a few days ago has tight bass and detailed mids and highs. It's a very smooth sound. Non-fatiguing.
Yeah sounds like it was a defective unit going by your comparisons between the supposedly identical units one without the gain jumper, and maybe the E1 in this review is also a defect