I have an SM10 and do not understand the audio level problem. I get it in theory, but I do not have the problem myself in practice.
As for the audio hum when hooked up to bike power -- I have heard the Freewire is built to run off 12VDC and has solved the problem. The SM10 the way most people use it runs off 5V USB stepped down from 12V in the acc outlet by the USB adapter. For me, that hums, so I have to either run it off its battery or charge it from an external power pack rather than bike source. Either works.
I saw the Sena guys (real Sena guys, not some local installer) at the Timonium motorcycle show and asked them about that. They gave me a very disappointing hand wave about the whole thing. What's worse, the harness they showed me for the Freewire had only the audio inputs, not the power ones, and the guy was saying it was an either/or thing. I could have sworn it was supposed to be both. Set my confusion level up several notches.
I would buy my Freewire from Sierra Electronics and ask them your questions. They seem to be a good supplier and very knowledgeable on the support side.
I will stick with my SM10 as is for now. If someone ever swipes it, I might just make do with the 20S FM radio. I use a cheap BT dongle as hack for the RARE times when I want a little CB. If that every changes, I will buy a Freewire from Sierra and sell my SM10.
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