Hi. If you are using the headphone amp in the SN3, are the speakers muted?
Yes, once the headphone jack is plugged in, it mutes the speakers.
Operating information to answer this is in the manual:
4.5 Headphone Output
The SUPERNAIT 3 headphone output is able to drive most headphone types. The 6.3mm (1/4") jack socket, rather than the smaller 3.5mm jack socket, provides more reliable connection and potentially higher sound quality. Most high-quality headphones are supplied with an adaptor that enables use with either 3.5mm or 6.3mm sockets. Inserting a headphone plug will mute the SUPERNAIT 3 speaker outputs.
Removing the headphone plug will restore the outputs.