From manufacturer's manual:
The RED BOX was the first speaker simulation DI box and a major success the world over. Now we present the new DI box generation, a totally reengineered successor: the RED BOX PRO. Filtering capabilities have been developed to the next evolutionary stage. The RED BOX delivers authentic guitar cabinet tone, effectively alleviating annoying and time-consuming miking tasks.
You can patch your guitar signal via the amp's line out or speaker output. When you chose the latter option, ensure you connect a speaker cabinet or some type of load resistor to the RED BOX PRO THRU jack. If you operate your amp without a sufficient load, you may damage your amp. The power amp stage in all-tube amps is a vital sound-shaping component, so if you are using this type of amp it is highly
recommended that you use the speaker jack rather than the line out to patch a realistic signal to the mixing console.
Box not included