I've emailed back and forth with genka and I've done the mod myself and here is the collective update notes:
- The way genka did the wiring (in the OP), the L and R channels are reversed.
- Also, the stereo separation between L & R is very poor.
- If you leave out the resistor, then all you have to do is turn down the volume when using ear buds and it will not be too loud - neither your music nor the Sena prompts. But, as noted, yes, if you use the built in speakers and have them turned way up, then plug in ear buds, they will be VERY loud until you turn the Sena volume down.
- In order to fix the L&R reversal, just swap the routing of the two wires that go to the L+ and R+ terminals. I think it makes it easier as the R+ becomes very short and no wires have to cross over each other.
- In order to improve the stereo separation, add a 4th wire that connects from R- to either L- or the Ground terminal (which L- is hooked to).
I did the initial mod, with no resistor and L&R preserved, and it works fine. I could not tell there was a problem with the stereo separation until I played a test MP3 that actually went full L, then full R. genka said the same thing. He had never realized there was a problem there until I reported it and he tested it himself.
I have not done the additional mod of adding a ground to R-. genka did it and reported the stereo separation to be greatly improved.
Here are a couple of pics of my version of the mod. No need to point out that I am a complete HACK with a soldering iron