Simultaneous operation of REC OUT and SPEAKER OUT?
The AMP1 was designed in such a way that REC OUT and SPEAKER OUT can be used simultaneously. You get quite interesting results if you combine your microphoned guitar cabinet (and alternatively or additionally the signal from the BluBOX) and the REC OUT signal. 
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By the way: The AMP1's REC OUT is fed by the SPEAKER OUT signal. So all the AMP1's stages affect the REC OUT signal. In addition, the settings of all volume controls, especially that of the MASTER control, affect the output volume of the REC OUT.