Just a hunch, but it could have to do with the nature of Hi-Z preamps. I remember reading that Hi-Z preamps don't require a transformer in the front end (or something like that). Don't DI boxes incorporate a transformer? That may why they squelch hum. May be better with the polarity switch flipped? Don't no. I haven't had occasion to use one. If an Ebtech device (which one I couldn't say) doesn't solve it, using quality DI boxes might be the answer for certain devices.