Hello, everyone! I'm looking to buy a Marshall Code 25, and I've heard that plugging a bass guitar into a guitar amp can damage it, therefore I'm wondering if plugging a bass into the amp and recording it through the USB port will harm the guitar amp.
No it wont damage it. I do it all the time.
An amp is an amp, the only thing that makes an amp specific to a certain instrument is frequency respone and more importantly for bass speaker design.
As long as you keep the volume low enough it will be fine (although maybe not as low sounding as a proper bass speaker). Certainly recording through the headphone or USB port will be perfectly fine as long as you can get the sound you want from the amp.
I have actually put a Bass preset on the presets section that you can download. Once again if you play through the speaker just keep the volume to a low level suitable for practice or recording.