Is this possible? I know that you can sort of achieve this with the lower models that have an MP3 input jack, but what about my model that doesn't? I swear I read something somewhere about bypassing the modelling part of the amp...
just leave it - it will ruin any sound you have anyway

just like Marshall AVT cabs....
You can completely disconnect the speakers and use them as a external cab. But dont think there is a way to "splice" in a MP3 jack on a spider 2.
I assume, then. they expect you to plug your MP3 into one of their pedals or accessories and go in through the RJ45 pedal port?