I bought a Crate 4x12 cab yesterday in hopes of hooking it up through my Marshall 30gfx. The amp doesn't have an extra input for speakers so I thought connecting it through the emulated line out port would work, since the cab should act as a giant headphone, in theory. Turns out, doesn't work. But I just want to know if this is actually possible. I mean, it should have worked, in theory. Or is it possible my cab is busted.
It needs to be a powered output for the speakers to work, the only way to do it would involve unsoldering the connection to MG's internal speaker and carefully wiring it up to a jack or speaker lead to plug into the 4x12.

I don't know what the hell you were thinking in buying a 4x12 for a crappy 30W practice amp, but if you've already got the cab you've now at least got the option of saving up for a good amp head!