Check out Weber's MASS systems. They're essentially a power soak made using a real speaker motor so it loads your amp like a speaker will. The resistor is a great idea for doing signal testing because the resistor behaves the same based on the power you throw at it (within the resistor's power rating) whereas a speaker isnt linear like that. Plus they have a unbalanced line out by default but can have a balanced line out installed for you as well.
Good suggestion on the Weber units. They are very "musical" attenuators. I've tried them and they really do work well. Ordering one with the balanced line out and will try to drive a cabinet simulator/impulse and see how it all goes. Cheaper and less boxy sounding than the isos I can afford, so that's a bonus too.