I just did a test with
- Two F1s (F1 Model 812 with F1 Subwoofer)
- Two S1 Pros for stage monitors
I set up the F1s on the front edge of the stage on either side, facing the audience area. I put the S1 Pros behind the line between the F1s, facing back toward the performers' area. I used the S1 Pros in the horizontal position as stage monitors.
There were no other performers; just me being loud (with hearing protection).
I pushed the F1s hard. From the stage area behind the F1s, they were loud but as you would expect, a little muffled. That's because the speaker arrays were pointing away from me. I wouldn't want to perform like that without monitors. And that's why I was doing this testing.
The S1 Pros were clear and loud up to their limit, but not noticeably louder than the F1s. I didn't have a hard hitting drummer so I can't say if the S1 Pros would compete well with a live kit.
I had a brief struggle with feedback until I found the right position for the S1 Pros. Microphones with a cardioid polar pattern might have been better in this situation. Most of my microphones are hypercardioid or supercardioid. These can pick up sound from the rear, so you have to be careful where you place monitors.
I like the S1 Pros in the monitor position. They seem to cut through better than in the tilted-back position. And the horizontal dispersion is narrower, so there's not a lot of spill across the stage. The tall vertical dispersion is helpful so you can have the S1 Pro close to you and you can still hear it at ear-level.
The S1 Pro is moderately loud (103 dB SPL, 109 peak), but not louder than what you hear behind the F1s. It's hard to express what how that will sound to you when you try it. If you're looking for monitors to use with your F1s, I encourage you to try the S1 Pros to hear if they will work for you.
From what I heard today, I would use the S1 Pros for stage monitors in most settings where I would choose to perform. Not-quite-so-loud and clear beats just loud - at least for me.