The above remarks around the Nio are one reason I am personally not too big a fan of the FIR O. It brings the mids and treble forward quite a bit - not aggressive or sibiliant, but markedly more in-your-face than with the M15. The uptick in volume necessitated by the reduction in bass (to get the kick) combined with that forwardness is a little fatiguing.
So volume needs to be up to get the kick with O module; and volume needs to be up to hear some of the detail with the M15. Hence my personal view is that it is indeed a higher-volume IEM. It still sounds great at lower volume but not its best.
So volume needs to be up to get the kick with O module; and volume needs to be up to hear some of the detail with the M15. Hence my personal view is that it is indeed a higher-volume IEM. It still sounds great at lower volume but not its best.