It might not help you, but did you try setting your USB headset as default in pulseaudio? When I want to use my USB headset with any software, I set it as default, then switch back as I want.
I use scripts with lauchers for quick access rather than opening pavucontrol
for my headset as default:
#! /bin/bash
echo "set-default-sink alsa_output.usb-Creative_Technology_Ltd_Sound_Blaster_Tactic_3D__Alpha_00203516-00-Alpha.analog-stereo" | pacmd
and for the soundcard as default:
#! /bin/bash
echo "set-default-sink alsa_output.pci-0000_00_1b.0.analog-stereo" | pacmd
I have also set the following in /etc/pulse/default.pa
load-module module-stream-restore restore_device=false
so that pulse doesn't insist in sending sound to the last device used with a given software.
To get the sinks name (as seen in the scrips), you can use the command:
pacmd list-sinks
I hope this helps.
Edit: Come to think of it, many complain about KDE's sound management, so this probably won't help. Sorry, but that's all I could think of.