I really like my audio but it was not a cheap or easy solution. And it was a process that evolved over the years. 4 Alpine speakers after burning out several other brands; 400w Class D Blaupunkt Amp, preamp with mixer, power filter, ground loop isolater; supports any audio device with mini-stereo plug, and has a dial volume control thx to the pre-amp mixer from Amplirider. No shortage of volume at any speed. No distortion. I'd like some serious bass but that requires a big speaker box; have to live w/o that - it's a tradeoff. Besides the thump-thump-thump from the punk cars - don't want to join that annoying crowd. I really like the Zumo 665 with Sirius tuner, doppler radar & traffic screens, and also plays mp3's. Limited to 2000 tunes on one SD card, but you can also put all of North America map on it, and set up more than one card if you think you'll get tired of a 2000 tune mix. I have tested up to 32GB card - that works, you can't run out of space.
The Zumos support bluetooth though I have never set that up. The mixer supports 3 audio devices; handy if you don't have them mixed by the audio player (Zumo does - music, GPS prompts, bluetooth). There's also radar, if you can't hear it's speaker (mine is loud enough; a Passport Escort).
I have not set up a helmet with speakers. In some states that may be illegal (headphones are some places) but who cares about that if it works for you. I'll say for me, I can hear traffic OK with the speakers on the bike; haven't tried helmet speakers. No doubt better fidelity on the latter. I'll say, having a volume knob is valuable for me - hate the menus things to control the volume - takes your attention away too long.
See
http://www.horseapple.com/Valkyrie/Tech_Tips/Jammin_with_Blaupunkt/jammin_with_blaupunkt.html