Routing Inst-IN-Chanel (Guitar) and USB1/2 (Mac-Audio) to MainMix/Speaker without DAW?

Hi, I have a new Audio FUse Studio and didn’t understand its Mixer/Routing.

I can only hear Inst4 when I solo it in the Mixer or push “Listen” on the interface. But I want to hear USB1/2 at the same time like Inst4 (without DAW!). For e.g. I want to jam along with the Guitar to a youtube jam-track without using any DAW or software. How to do that? The only way I figured out, is to do it in a Cue-Mix. But that reduces the Level. Why is this? And how to do this in the Main-Mix?

Andy why ist there no Master-Signal via Main-mix to the Speaker-Outs when USB1/2 is not in Solo? That’s confusing.

And why is there always a little singal (about 10%) at the digital out SPDIF?

Please help me out.

Would send a pic how my Mixer is set up, but seems not to be possible in this forum?

Thanks
Tim