sick rhubarb?

Something is wrong with my Rhubarb! It's fantastically huge (the fence behind is four feet tall), but starting to look sad.

The lower leaves are turning yellow and brown, lower foliage has holes, and some have these ugly spots, too.

I check throughout the day and in the middle of the night by flashlight, but find no bugs on it except a few fireflies and earwigs. Do earwigs eat rhubarb? Are my plants just undernourished? Does it need more compost? more sun? I don't know if I should remove the damaged leaves, I'm afraid to take any off and weaken the plants.

This one looks like I planted the crown too deep; the lower leaves always end up lying on the ground:

Rhubarb, as far as I know, doesn't have many pests or diseases, so what's wrong with mine? Help!