Hi Trumpet52
Welcome.
Ordinarily I'd assume the trumpet, which puts out a lot of decibels in front of a mic, is overloading the Yeti either at the diaphragm or its preamp. The output adjustment on the mic is just to adjust amount getting into the DAW, after all of the mic electronics, I'm pretty sure, like the output on a mic pre that also has an input knob. The Blue website claims the output knob changes mic sensitivity, but I'll bet it's just the final output and not actually the sensitivity of the mic (which implies before the output). But when you say it does this even when you stand over 8 feet back and when you play softly, that very likely takes that out of the equation. What exactly does the soft playing that's peaking sound like? Is it peaking in Reaper's track meter on playback? If it doesn't, is there any chance this is a playback thing and not recording (given that soft playing is doing it?). Is it distorting or just visually peaking?
Either way, definitely lower the Yeti output gain down to its lowest and compare that.
What computer OS are you on? I can't speak to Windows but on Macs if you go to the sound system pref panel you have settings that include input and output level, which will affect USB mics. (Make sure you have the Yeti selected as your USB sound input device. It won't be the first time one of us has been fiddling with a USB mic and recording with the computer's mic because it was the selected input
)
Do you have the Blue Sherpa mic software app?
https://www.bluedesigns.com/products/sherpa/ It doesn't apply to all of the Blue USB mics (maybe not all of the Yetis even) but it gives control and might be overriding the computer settings if it's installed.
Also, how are you monitoring? Through the Yeti headphone output or the computer's? If all the previous bits about the input don't change the problem, check that it's not just in the monitoring. You said that the track is fine but the trumpet isn't. Do you mean even when the trumpet is soloed? Or just when you have the new trumpet with the track? It's not impossible that if the bass and drum tracks are near maxing the master fader in Reaper that adding the trumpet is clipping the master. Or even if not that it's now clipping the headphone input (in the Yeti or computer) when it wasn't before. What happens if you lower the headphone volume? Or lower the sums, bass and trumpet tracks in Reaper and adjust headphone to compensate? Have you tried singing into the mic, instead of playing trumpet, with the same tracks and seeing what happens? Trying to determine if this is trumpet/loud source specific to the Yeti, which is more set for voice, or if it's not the Yeti that's the factor.