I'd be asking questions about how a given timber reacts to humidity changes myself. Here in The UK, you get little variation and what there is is slow to change, hence very few people buy humidifiers for their guitars whilst they are far more common in The USA.

My gut instinct with tops (I'm not a luthier but used to design vibratory equipment for industry and wrote of guitar design in comparable terms for my doctoral thesis) is that a good straight grain allows transmission as opposed to overall deflection. However, the success with Koa and other knotty swirly timber tops makes me back away somewhat from this stance. A guitar builder in the next village whose latest offerings usually get played in by me (as well as having built me a stunner) has just completed a mahogany guitar and the top, as well as the rest, can only be described as "quilted." Two weeks old and sounds so good already I may grab it before it hits his sales stock. (Reiner Guitars.)

Just one more thought regarding underside of top, back and sides. The rougher the finish the more surface area to dampen sound waves. Ditto pitch of fibres. I reckon the finish inside is as important as the choice of timber. Cheaper guitars with ply back and sides are also generally less well finished hence quieter. Big name guitars with a ply back n sides range (Martin X, Taylor 1., etc) are good sounders. Why? I reckon the internal finish is an important factor.

I''m fascinated by the subject and have sent a link to this thread to my friend. He only uses spruce and mahogany for tops as cedar gives him a rash when working with it. I mentioned hemlock and that got him thinking...