Did not analyse the circuit of the iso-x yet but with standard studio line level you can not even push the 6dB without heavy distortion. And also with no gain the sound is really not good. Could send some sound files later if anybody is interested in.
but I believe it's easier to design a discrete circuit that doesn't mess with a signal's phase than an IC circuit.
Interesting that you say "easy". I build my own mixer using standard NE5532 or OPA604/OPA634 ICs. For me it's easier to build up a simple IC designed circuit. And if you take care of a good supply (various capacitors near by every IC voltage input), short links at important places in the circuit and good elements (maybe avoid electrolytic capacitors?) you get a sound and measurement results that beat every mixer.
In my experience!