I’d check a couple of things:
1: In the Devices /vst menu check the "Direct Monitoring: box. (this should ensure that pressing the Monitor button on a CB mixer track activates audio pass through on the 816.
2: in your VST Connection make sure that in the “Outputs” tab any entires listed there are set to ‘not connected’. If you are using the Control Room feature you want to assign all outputs there. (in fact it might be good for SB to disable the ordinary outputs if the Control Room is in use … though there might be use cases I haven’t thought of).
3: In the Control Room Tab make a Monitor Buss for your Speakers (you’ve done that . and assign the 816 outputs connected to the speakers to it). The level of the speakers should now be controlled by the CB Control Room level, AND the 816 Master Level knob.
4: In the Control Room Tab create a “Studio” buss (this is for artist Cue feeds) and assign it to an 816 physical Output ( that is not connected to something else … I ofter use one of the ADAT outs. Again it would be nice if SB offered the “Phones 1/2” as output options here directly, rather than eating a physical output for it). Now go to the Cubase Devices/Audio Hardware menu and select the Phones Tab. Here you’ll see the two main Headphones levels … click above one of the knobs and select the physical output you assigned to the Studio Send to the Phone output. (phew). Make sure the Phone knob is turned up … note that this knob is a mirror of the one on the front of the 816 … if you turn the real knob, the virtual one should follow.
5: Now go to the CB mixer. If you have the Input channels showing you should right click on one and choose to see 'Hardware". This will reveal the 816’s reverb sends, and voice channels (if you have the csx model). In your Audio Track section choose the track that the mic is being sent to. Click on the Monitor button to enable direct monitoring.Make sure the extended mixer is showing (so you can see inserts, EQ etc) and on the left view panel click on the Star icon. Now you will see the the send level to the Studio (ie Phones) you set up before. There will be 4 of them, since that’s how many Studio Sends the control room allows. Turn on #1 and raise the send level. You should now hear the mic input in the headphones. Adjust other Audio tracks send levels to create a mix for the performer. As the engineer your mix on the speakers is controlled by the Channel faders of the mixer. Make sure that the Studio Sends levels are set to “prefade” so that the main Fader levels of the mixer don’t adjust what the mix the performer is hearing… there’s a button next to the Send Level which toggles the send Pre/Post fader.
6: Of course make sure the 816 is set to Mode = Internal FX (in the Cubase Devices/Audio Hardware menu) otherwise none of this will work!
Once it’s all set up I do a “FIle/Save As Template” so I can load it in for tracking really quickly…
When you get used to it, note that the Studio Send levels can also be accessed from the Track Inspector. So you can close the Big Mixer and just select a track to adjust it’s send to a performer. Less window clutter that way.
Hope this helps