Just received the sennheiser ie 100 pro IEM’s for use with the M8. Only complain i have is why i didnt order this earlier 
Of course, dont expect JBL Harman sub bass (fake boosted eq anyways). Good Bass is there when you use the correct tips for your ears.
These sennheiser’s has a great sound stage overall and can get very loud. Also very well suitable for long sessions because of the comfort.
Good price too.