I just remembered that my daughter had a little Samson USB Mic, so I thought I'd give that a go just for grins.
It's too late to do any serious measurements or super accurate mic placement (and I forgot to save the couple measurements I did do), but with the crossover in place (no C1), more or less on axis, roughly 18" from the CD diaphragm, the freq resp from 3K-20KHz is actually tipped up!
More measurements tomorrow. In the meantime, I'll resist the urge to throw my USB mic pre in the lake.