One thing, please - Pres never sounded "like an alto". His "light" tone was actually the result of a darkening of his sounds' upper partials, as well as sending a "wider" air-column (best way I can describe it) through the horn (but still supporting it fully, that tone of Pres' ain't gonna get swallowed up by anything).. If anything, his tone was less like an alto, although if the only real point of reference was Hawk (or the beloved Bud Freeman), then they wouldn't yet have the baseline to figure that out. But even in his early days, Pres's tone was always "darker" than Hawk & Co., because of the de-emphasis of the upper partials of his sound, which is a fundamentally different thing than upper register of the horn. Hell, you could say that it was Hawkins & Co. who were more "alto-like" in that regard!
Whitby, it sounds like to me, has as bright a tone as the norm of the time, but not as "full". This is probably the result of using a setup that still emphasizes the upper partials in the tone, but using a less dense airstream. As I've said before, it sounds to me as if his embouchure and concept of airstream is rooted in clarinet playing, but maybe not.
Tone is one of those things that stir a wide variety of emotional responses (just as many people think of Coltrane's tone as "dark as they do of it as "bright"), but the physics of how any given tone is achieved are pretty much objective.