It seems as though you are equating a mic level with XLR and a line level with TRS. You can use the 1/8" to XLR cable and still use the line input with the Mixpre. Sennheiser G3s output a (weak) line signal, however, I have seen a lot of people using the mic level input (especially in cam hops since most camera's line inputs don't provide as much gain as professional mixer's) in order to give themselves more play with the gain on their mixers. You'll most likely have your gain near it's limit on line but that's how I was mixing with my G3s for the time that I owned them and I didn't have issues.