For sound isolation, higher dB better.
Impedance i dunno how to explain.
Higher the value, the harder it is to drive.
ER6i is L shape but the ear piece is small, can fit thru there. Too bad i sold my ER6i, if not, i can try on it. ER6i can be worn upside down with the cable going over the ear and back. SQ wise, no diff. I wear them this way but with the cord in front cos that time, i got no heat shrink.
Filters are made of hard plastic (cannot stretch) and have different qualities, so its probably not interchangable.