I set my Garmin 665 to 100% volume and use the knob on the sena to "dial in" the sound level. I know it seems like a little thing, but I would recommned/buy the unit just for that. trying to turn down the volume prior to the sena was a pain.
One thing to note about this. The Sena has built in volume memory as well. You can individually set the volume for your Music/Garmin/intercom completely separate from each other and it remembers the volume setting for next time it connects. That way you never have to worry about turning up the volume in intercom mode and then having your ears blown out when you switch back to music. It will remember you want (for example) music volume at a level 3, and intercom volume at a level 6. Everytime you switch, it will automaticly adjust the volume to match what it was the last time you connected to that profile.
-Keith (Yup, thats me...and my rubber chicken)
Making the world a better place, one rubber chicken at a time!
'98 Buell S1 Lightning
'00 R1100RT (parting out, PM me)