On the Code 100 you have a 1/8-inch aux in jack which is usually designed to accept headphone output from cell phones, iPad/iPods, mp3 players, etc.

Always good advice to start with outputs as low as possible but the power amp output and the headphone output are likely independent of one another (but both controlled by the preamp section).

The first question you should ask yourself is: Do you like the sound of your Bugera through headphones (i.e., the headphones jack)? If not, you're likely to be wasting a lot of time and effort with GOGI/GIGO kludges. A mixer would probably be the Swiss-Army-Knife solution but don't be disappointed if it doesn't cut as surgically or as deep as you might expect---you're introducing, for better or worse, even more circuitry and processing into the signal path. A Red Box is a DI designed to do what you want.

There are DIY ways to connect your Bugera directly to the Marshall's speakers but it WILL cause damage if not done and used properly and, given your expressed understanding of I/O (no disrespect intended), I'm not going to advise it.

Paj
8^)