| The biggest issue is weight, so anything made of wood will certainly have less impact than would a brass extension. The difference in weight of an extension made of maple with an ebony top vs. solid ebony is less significant than the design of the extension itself, as some luthiers build much more massive extensions than others, and the geometry of your scroll will also play a role in how it must be built. Stiffness probably also plays a role, but again I doubt the difference in sound between solid ebony vs. maple/ebony based on this parameter is very great. At least I've heard no such claims.
One thing to consider though is that any extension is going to affect the sound of your bass somewhat, and the effect of added weight to the scroll is often positive. Traeger's book describes a clever trick: clamping weight to the pegbox, based on the predicted weight of the extension. It isn't a perfect indicator, but a very easy way to get a basic impression. An off-the-top-of-my-head estimate of the weight range of popular wood C-extension designs (without latch hardware) is between 150 and 250 grams. By doing a little experimentation at home, you can decide which way you want to go, and your luthier can choose woods and design parameters to build a heavier or lighter extension.
Hope this helps!
__________________
Robobass
| 
