Re: Simple load box to attenuate speaker signal
I'm going on record as saying this is a bad idea, you would be better off finding one of the Mustang floorboards since those are made for exactly what you want to do, or possibly finding one of the bigger Mustang amps so you don't need a 2nd amplifier.
FWIW, the definition of Impedance is the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.
That said, it's a ss amp, so you can run it without a speaker hooked up and it will be fine. Or, pretty much any impedance above the minimum rating (probably 8 or 4 ohms), and since it is SS, the higher the impedance, the less power it can put out. And yes, a resistor provides a frequency independent impedance, meaning it is the same DC or AC. A SS amp sees nothing hooked up as an infinite impedance (or resistance) so it can't really generate any power without a speaker, whereas a tube amp without a speaker only sees the resistance of the primary coil in the transformer, so, without a speaker is sees a very low reistance/impedance therefore tries to generate very high currents, which turns into heat being dumped into the transformer, and eventually burns up the transformer.
So, yes, if you use resistors that can handle the power of the amp you could theoretically do what you are wanting to, but even a 50W resistor will get hot with 10 - 20W being constantly dumped into it, you'll probably want to do some heat sinking or something, but again, the more resistance, the less power the amp can put out. Still, taking that into account, see my first statement.
*EDIT* Going back and reading, you've already built it and it works, cool. You could have used a higher resistance (impedance) than 16 ohms and be fine, it will just reduce the amount of heat that is generated in the resistors. You could also probably make a voltage divider circuit and tap off of that to feed the other amp. Or, as Surgeon suggested, an L-pad would work nicely although I would still probably put a resistor in line with the tap feeding the 2nd amp's input.
Don't take this the wrong way, but you seem to have a very basic understanding of electronics, which in this case worked out okay, but I would definitely recommend a better understanding before digging into higher power/voltage circuits. At the same time, a lot of the advice you got in this thread, while well intentioned, wasn't exactly correct for the application either. Tube amps do care a lot more about how the load (speaker) reacts with the transformer, but a simple ss power amp like in your Mustang is pretty straight forward and doesn't really care as long as it sees a resistance/impedance higher than it's minimum rating.
