this problem is not only a headset problem, its a over all volume issue (most importantly handset) I have that problem too, here is three possible solutions that worked for me, and "MIGHT" work for you.
Give these a try and be sure to give a feedback whether it works for you or not:
A) Enter service mode by typing below code on your dialer :
click the following:
 NB (VOICE CALL)
Choose Handset, headset, Speaker (which ever you want higher volume)
 SRC Speech RX Volume
Change default values and higher the values any number of your choosing, here is an example:
0_lvl : 68
1_lvl : 72
2_lvl : 76
3_lvl : 80
4_lvl : 84
5_lvl : 88 Higher these values to until hear volume improvement.
Can also edit the values from:
DST I2S1 TX Volume
SRC I2S1 RX Volume: 100
DST Speech TX Volume: 94
This is Samsung volume setting, worked for several people but sadly only worked 2 times for me and values keep resetting for me (doesn't mean for you, so give it a try) Also this is based on STOCK firmware.
B) the second thing you might to try is app called Volume+:
This app guarantees it higher the volume without the need of touching its default settings, BUT huge drawback it highers everything: Headset, speaker and Bluetooth EXCEPT HANDSET which is the most important :(
(I also downloaded many volume increase apps, but Volume+ was the only one got the job done for everything but handset)
C) Lastly get GOOGLE VOICE app (create a GV account if you don't have one)
just by installing Google and logging on, every time you make a call, your calls get forwarded through Google voice and volume levels become acceptable in ALL HANDSET, HEADSET, SPEAKER, BLUETOOTH (just like phone should of have been) (this what i personally use fix my volume issues)
Hope one of these work for you.