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The big enemy of classical guitar sound, IMO, is weight: you are trying to make the lightest possible top that will withstand the tension. There are two reasons for this:
1) the strings are under less tension, and so have less energy at a given amplitude, and
2) nylon just naturally 'eats' high frequencies. There is much less high-frequnecy energy in nylon strings than there is in steel, and you can't afford to waste any if you're going to have a 'crisp' sound. The key to high frequency response is acelleration: the top has to be able to respond quickly, and that means low weight. There are two major things to look at here (or, maybe, three, depending): density and stiffness (you can think of long-grain and cross-gran stiffness seperately).
Very generally, the long-grain stiffness of spruce will tend to scale with density. I just tested about twenty tops of various sorts, and the data points in a density-stiffness (Young's modulus) plot cluster around a line. There's a lot of spread, of course, but in general every type of spruce I checked, Sitka, Red, Englemann and Italian, was near the same line.
So, suppose you start with a Sitka top and a Euro one. Let's say the Sitka top is twice as dense as the Euro, and has a Young's modulus that's twice as high (which can happen). Bending stiffness goes as the Young's modulus and the _cube_ of the thickness. A given piece of wood twice as thick is eight times as stiff. To make the Sitka top the same _weight_ as the Euro one it will have to be half as thick, which would reduce the bending stiffness to 1/8 that of the Euro wood if they both had the same Young's modulus. Since the modulus of the Sita is twice as high it's going to be 2 X 1/8 as stiff, or 1/4: pretty floppy. So, all else equal, go for low density. This is why, in general, the denser woods like Sitka and Red don't get used as much for classicals.
Sitka has a couple of other things working against it. One is that, again in very general way, it has lower cross grain stiffness than Euro. I always look on cross grain stiffness as 'free': you don't have to add braces (or splay the fan more) on a piece with good cross stiffness.
Another peculiarity of Sitka is the way it absorbs sound. Most woods don't disspate much energy at low frequencies, but the amount of 'loss' rises as the frequnecy goes up, particularly above 2000 Hz. Sitka seems to have higher losses at low frequencies than most woods, and this loss _falls_ as you go up until, at 2kHz, it's about the same as other spruces, and it rises 'normally' after that. Nobody is quite sure why this is, or even if it's perhaps a problem with the measurement techniques. Still, it seems pretty consistent.
WOOD VARIES! If you've got a really light Sitka top that has high cross grain stiffness, then go for it. 'Bearclaw' adds some cross stiffness, but, of course, it mostly shows up on non-quartered wood. Still, if you can find a nice piece... BTW, the best way to know if you've got a nice piece is to measure. It's amazing how easily fooled your senses are.
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