The problem from my point of view is that you're starting off on nearly the wrong foot. It's a bit hard to explain in one post on a 'board, but I'll see what I can do.
'Warmth' is one of those tricky descriptors. Usually it's taken to mean a sound with a lot of low end, but I think it's more complex than that. Low end by itself usually sounds 'tubby' to me. It seems to me that for a really 'warm' sound you need that low end plus a lot of higher overtones. Your ear assigns the energy from those overtones to the fundamental, and that helps to 'fatten up' the sound, particularly if the high end peaks in the spectrum are well defined, with 'dips' in between, which makes things 'interesting'.
The way you get that sort of spectrum is to reduce losses in the wood. The 'warm' sound of rosewood has a lot to do with it's long 'ring' when tapped, a sign of low losses. Cedar also tends to have lower losses than spruce, and I think that has more to do with the 'warmth' of it than it's low density. I'll note that one of my recent classicals has a redwood top that is as dense and stiff as most spruce, but it also has notably low losses, and the guitar has the sweetest and clearest trebles of anything I've made.
That guitar, BTW, has walnut B&S. That was a deliberate pairing: I've found that most of the spruce/walnut classicals I've made have been overly 'bright' and 'forward'. I attribute this, in part, to the low density and rather high losses of the walnut, which gives the guitar a lot of 'punch' without a lot of sustain unless something is done to compensate. (Yes, I know I said the low losses of rosewood contribute to warmth, and now I'm saying that high losses in walnut contribute to 'brightness': I told you it was complicated. It has to do with the pairing of density and loss factor)
Engelmann spruce is often low in density, and can have reasonably low losses, but it varies a lot. I've got Engelmann tops that have a lengthwise Young's modulus (E) of around 8000 megaPascals, with a density of 360 kg/cubic meter, and others that are at 500 kg/m^3 with Elong around 17000 mPa. The low end ones are right in there with the lightest cedar, and the high end ones are more like dense Red spruce. The irony is that I got the dense ones when I asked a supplier to send me their lightest tops: they're all sanded thin, so...
Anyway: try to figure out what the density of your top is. One way is to wrap it in a plastic bag and put it in the water endwise. Mark how deep it floats: if 40% of it is in the water, the specific gravity is .4, and that's a density of 400 kg/m^3. One of the nice things about metric... If it's at the low end of the scale, below .4 sp.g., then I'd leave it about 10% thicker than C&N tell you to, and go from there. If it's light and rings a long time when it's tapped, then you might end up with the sound you want.
Oh yeah: use the spruce bracing.
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- just to have some contrast inside the box.![[:Y:]](./images/smilies/smiley20.gif "Thumbs Up")