20S EVO is no longer the winner. I am over Sena. End of story.

Ignore this post - he hasn't a clue...

Among other things, the 660's voice being infuriating loud, with no hope of changing the volume, put me past the tipping point. The TomTom Rider 550 turns out to have the same problem. But...
There is a way to change the volume with the 550. The key here is the 20S or 20S EVO has to able to handle two A2DP (the HiFi BT format) BT connections at the same time. I started working this thinking that only the EVO did the two ports thing. BZZZZT! Wrong-o Bluetooth breath. Upgrade the 20S up to firmware V2.0 and it becomes almost the 20S EVO without the antennas, etc. Including the two A2DP-capable ports. From there I did the work-around for volume control for the voice prompts. Woohoo!
Sena still remains mostly silent but the V2.0 update addresses a host of requested fixes and feature requests. Too bad the OM remains at 1.127 but using the EVO OM gets around that. In the end, I'm not over Sena after all.