I've not had a 20S but have been using the 10 for years with great success.
Two points to consider, turn up the volume on the iPhone AND the volume on the 20S. Note that when you turn up the volume knob on the 20S, you're adjusting the volume of whichever 'mode' it's in at that moment. If you're talking on the phone (HFP), for instance, and adjust the volume of the headset, once the call is over and the headset switches to music (stereo A2DP mode) you will need to turn the knob again to set the volume in music mode. The headset remembers the volume settings for each mode. You can adjust it such that the when the phone rings or you get a notification of a text, that it doesn't 'blow you away' but your music is turned way up.
Second, in the case of the SMH10, the original speakers suck. Several in my group have opted to upgrade by swapping in some speakers from a particular set of cheap Koss headphones and have found a huge improvement in sound levels AND quality. I do not know if the speakers of the 20S have been improved over the SMH10's.
(edit, related) The original speakers on the SMH10 are VERY "focused"... meaning that it is as if the sound is in a very concentrated beam...like a laser pointer compared to a floodlight. This (in my experience with the SMH10) meant that I couldn't hear it unless the speaker and my helmet were positioned *just right*. You can easily test this (when stopped) by grabbing your helmet while the music is playing and moving your lid around to see the audible effect.
My primary reason for doing the Koss upgrade was to fix this aspect; the Koss speakers (again, *to ME*) are more like 'floodlights' and are very forgiving on placement and direction. Oh, yeah, they also have bass. Very satisfying 15 dollar upgrade.