PC Master Out not working - DDJ - SX2

Hi, I've just bought a DDJ-SX2 and I'm trying to get the PC Master Out function to work, with no success.

I have the latest Rekordbox, and the latest drivers, and the latest firmware.

I have the DDJ-SX2 ASIO driver selected for the output, and the master and headphone outputs are working fine.

But when selecting a master output channel, I only see:

PIONEER DDJ_SX2 ASIO: MASTER

I do not see any kind of option for:

PIONEER DDJ_SX2 ASIO: MASTER + Speakers

So even though the purple PC Master button is turned on in Rekordbox, there's nothing coming out the default sound device.  All other audio in Windows (win 10) works fine coming out the default device, which is a Komplete Audio 6.

What can I check?

Thanks

Lindsay