Studio One's monitoring facilities are quite limited. Below is one example of how studio monitoring should probably work, using Ardour 4. Thanks for reading or voting.
Solo In Place
What you see below is a channel in solo and in the "Solo in Place" mode and it behaves like S1 and most other daws, it mutes everything else, including the send from channel 1 that's feeding headphones. One other thing to notice there is that muting doesn't stop any metering...
If you switch to AFL or PFL it behaves differently. Solo still solos the signal but it doesn't mute anything so my cue feed to Phones 1 is still going. So in that regard, I can solo signals pre or post fader / FX in the mains - while recording - to isolate things in the mains, without interrupting the cue sends to performers.
Building A Cue Mix
Below what you see happening is me soloing that cue, "Phones 1" (assume it's feeding a hardware output). Take note that it doesn't stop anything else from playing but I can hear that mix in the main monitors directly while adjusting that cue mix. Also, take note that I added another cue "Phones 2" to help illustrate that soloing the Phones 1 bus to adjust that mix in the mains doesn't interrupt other cues.
So to adjust the Bass players mix, I solo that bus and adjust sends across the mixer to it, and hear your adjustments directly as I go. Take note that the main meters aren't showing the solo channel signal alone, it's still metering the full main mix. The soloed signal is being bussed directly to the mains...