Never really understand when people say they feel like listening to speakers or check whether their speakers are on because it sounds like it.. Speakers and headphones are worlds apart for me no matter which setup I have listened to. My Stax Sigma reminds med of a (very) well acoustically treated room with speakers in it.. But still, worlds apart.
I can kinda relate to the headphone 'dissappearing' as you zone out, though.
If I had to describe it more accurately, I'd simply say the stereo width has drastically increased compared to the EF2a, I don't think I'll mistake which item I'm listening to, but these sound like speakers sound, the sensation of direct sound pressure on my ears is diminished, it feels like the drivers are not struggling to get the desired excursion, the ef2a was very likely under driving them, and it's literal gobs of gain was bringing it to listening levels. That doesn't seem to be the case any longer, could I say it's directly attributable to capable power or balanced? Well one seems more likely, I couldn't care less about balanced other than I fell in love with the Mjolnir the first time I heard it and had to have it thereafter.