Some excellent suggestions here, thank you!
I am going to investigate the passive DI box first, which I have to admit I have never come across before (I'm old school, 62 years old... LOL). In answer to "Why not just stick a mic in front of the Orange Crush?" – which was my first reaction, the answer is that he wants a very clean signal so that he can do all sorts of stuff in the desk without background noises being picked up. But yes, that was my first reaction.