WOOD OPTION

Engelmann Spruce Picea engelmannii is the softer of the spruces and can produce a more immediate bass response, particularly when combined with the "X" bracing pattern and a red maple back. The characteristic tone is typically warmer and thicker than the harder spruces such as red spruce that produce a stronger mid-range response.
BRACE OPTIONS

European Spruce Picea abies is a little softer than red spruce with similar properties to engelmann spruce, but trees with harder "compression" growth rings produce many of the same qualities of red spruce and is a good alternative, particularly for larger instruments such as guitars where wider billets with even growth rings are harder to source in red spruce. 

Red Maple Acer rubrum is similar in appearance to sugar maple but not as dense.  Generally it has a figured pattern that is broader and more regular than sugar maple.  It works well in combination with the softer spruces and the X bracing pattern where the maximum bass response is preferred.