There already is one.
There isn't any number lower than 0 for gain in terms of gaining.
The E12 wasn't mean for IEM's.
You can use them with it, but some may hiss, others may be too sensitive.
E17 will work with more IEM's.
Don't worry or worry less about that.
The E12's volume pot was either designed or was by chance good already in that.
It is extremely hard to turn the pot when you have only one finger trying to turn it. IT is a bit sticky during the lower turns but gets looser as you get up top.