Cool, thanks for the info. I'm going to get the 91 eventually I think. Apparently Jim Stringfellow makes an adapter thats more hydrodynamic than the original one. I'll be keen to hear how it goes once you get it.
The dudes at Mackiteboarding reckoned they could notice a bit of a hum at higher speed but not an issue for cruising around.
Didn't see the Stringy one - a bit more rounded. Too late now!
I'm not getting any humming from mine. All I can hear is the wind on my lines. But the 683 is not built for speed though it can carry a fair bit is your keen.