Yup that pretty much sums it up! Speaker outputs are high level outputs (I.e. higher than any interface could ever handle ever). The only way to do this would be to plug in some kind of device that soaks up the power and matches it to the level of your interface. You would save a lot of time, money and thermonuclear disaster scenarios by the following:
A) Get an amp sim like a pod, POD Farm software or similar.
B) Get mics and learn to mic cabs.
C) Look at actual hardware simulators that are designed as I mentioned above. Google Palmer PGA04 for an example of what I'm talking about.
D) Get a good quality DI box and get someone to reamp your DI's.
Remember always...
Speaker output -> Line in -> Byeeeee....
Also bear in mind that to operate safely, power amps need a load to dissipate the power they generate. If you look on the back of your head, it may something like 8Ohms load min. This means that anything connected to it must have an impedance value in Ohms that exceeds or is equal to 8Ohms.
To be honest you will save yourself a lot of trouble to get an Sm57 off the net and learn to mic it.
