Check the moon phase. If it's anywhere near a full moon, you won't need a headlamp at all really (still bring one, but don't worry about getting a brighter one). One of the best experiences is hiking above treeline by moonlight - all that granite reflects the moonlight and you can see everything so bright and clear.
Always bring warm clothes (like your rain gear, it's a safety thing), but I doubt you'll need your base layers unless you plan on spending a lot of time sitting still on the summit. Last time I climbed whitney in july (last year), I was wearing shorts and a thin t-shirt at 2 AM on my way up from guitar lake. Along those lines, I would suggest leaving the long underwear at home unless you are only planning on wearing it to sleep in.
It starts out cold, but, I don't know if you noticed, climbing, even up an easy trail like whitney, is pretty physically demanding, and you will end up sweating your butt off if you wear all those layers.
Have you ever climbed Whitney before? Try and make it up for the sunrise =0)