It kind of depends on which Marshall you have. If it’s one of the old Super Leads with 4 inputs and two volume controls, then you could theoretically plug your music source into one of them and your guitar into the other. However, your EQ will be compromised between the guitar and the music source.

If you only have one input, you could use a small external mixer to combine your guitar and music source, and then send it to the amp. However, that would require you to run the amp very clean. So, if you’re counting on the amp for distortion, this option is out.

There are several practice amps out there now that have an input for a music source that bypasses the guitar preamp and EQ. So, you can have your guitar distorted and your music clean. Fender has a couple of amps like this now, as does Line 6. If you’re looking for a 1-box solution, this is probably your best bet.

Good luck with the audition.

Greetings from Austin, TX

Ken