I agree, numbers would be great. I find them appropriate for digital
work, and they're very fast to read. In fact, you could have two
numbers, one for the value of the ex-knob/slider, and one for the value
of the level it modulates (the mapping may not always be linear ...).
Additionally you could have a coloured background behind the number.
For me, knobs and sliders are mostly useful in hardware because they're
fast to manipulate and tweakable and you have tactile information about
their values. I'm 10 times slower with a mouse and pixels.
And the liblag idea sounds nice!