I would just like to say thank you for recommending the ER4S - they sound absolutely amazing, and found the bass to be excellent, and have not experienced sibilance!
They have much wider soundstage compared to the RE0 and CK10, not quite the same with height and depth, but still good. I wouldn't call these bright phones - more refined, neutral, and balanced. My only real gripe would be the fit - though custom reshell or sleeves would solve that. Unsurprisingly the ER4S can't quite compare to the RE262 in terms of presentation, well at least in terms of absolute measure. The ER4S however has much better imaging, instrument separation, accuracy, and resolution. Overall I would call these an upgrade from the RE0, and sidegrade from the CK10.
Thanks again Joker!