one problem here is volume matching. I play with little impedance adapters all the time, but it's really a problem to have so much difference in volume (going from nothing to 100ohm is no joke in level attenuation). so the ultimate dreambox would need something to compensate the different volumes for each impedances, but that part mustn't change impedance by itself... argh!
also depending on what IEM you plan to use in the futur, lower values can be nice. for an ER4 5ohm is a joke, and for W4 the dip in high mids is so huge that anything is welcome to reduce it, but some IEMs are super sensitive to impedance, yet really close to perfect(for me) signature wise. then just 2 or 3 ohm can be the icing on the cake to just make that 1db difference.
also I find it usefull when making 2 sources close in impedance output so that I can judge real differences.
anyway, if you make such a box, I want one! (unless you make your price tags like astell&kern)
edit: ahahah same ideas same critic, I love it!
Haha. I'm thinking of making it. I already have the parts list. I'm guessing it would probably be somewhere between 40-50 dollars at most (12 resistors at $1/ea., wire, 2p6t rotary knob $6, 3x 3.5 jacks $5/ea., 2pdt switch $6, $3 heatshrink, case $?) for me to make myself not including a case. Not sure when I'll get to it, but I'd like one for myself, as impedance changes a lot of iems. I wouldn't be using it to "get the perfect sound" with the impedance, but rather to see the general effect impedance has on a certain iem. Then I could always custom make an adapter with the specific impedance I find best for that iem.
The other option is to just make an impedance "box" without the switch, as I already have a switch anyway. This would save half the cost almost, be more portable and be useable without the a/b switch to test anything. That might be better for now anyway. I could even throw in the capacitor to make a setting for turning the er4p into the er4b.