The bass is definitely the weirdest/worst on BAs but treble and mids also lack some definition. Like part of the textural detail is smeared and when you play close attention to it you kinda hear some grain but the FR is so smooth it doesn't sound very grainy overall.
I want to hear the ER2 but I'm afraid the tuning of the ER4 is too good even if the ER2 is a slight downgrade.
The FR of the ER2SE is the same as the ER4S. Just has dip at 8k and 12k, But still has more treble than the 4SR. I agree with the bass bit the ER2SE outperforms the ER4, i find the mids/treble are better more so with -3db at 1.6k & 6k. Also a 2.5db high shelf gives slight more sparkel above 8k.
Amazing that they did this with a 5 mm Dynamic driver.