If you are not using member ninja adapters, then is will happen.
Even the cheaper 3d printed version I have also does not keep the bass..
So you are correct about the duller and lack of bass in those cases.
The Sine has an issue about ear proximity.
It needs to be close and have an excellent seal or you will loose bass and get a dull sound.
I had most optimal results with the combo of fake MSR7 pads, bose lettered felt covers, and the ninja adapter...
Better than the Vesperaudio pads...
Yet I sold my sine and moved on because I was never satisfied with both extremes of bass and treble.
Then I was quite shocked when I thought it was so great quality and my headband snapped out of the blue, just by stretching open to lower onto my neck.
Once I received my perfect new replacement, I sold the unit because I need much more durability as I am type that use everyday and most of the day even during work!
The Hifiman Edition S is no better at all in durability and its headband snapped quite easily in dead center when I pinched it closed with two fingers!
I assume the plastic composition may have been corrected by now as mine was an very early unit...
Anyways that's my two cents.
I personally have settled on having to use EQ on another portable to get what sound I want.
I cannot recommend any other headphones that the two mentioned at the moment regardless because they still have best sound in Portable use.
So take care of your Sine, and DON'T STRETCH THE HEADBAND!