If you're asking me, I'm using alsa, not pulse.
using pavucontrol or not
I also use Alsa ..... PulseAudio does not replace it.
As far as I am aware you have three choices ....
1. use Pulse Audio so you can select the desired streams through the GUI
2. use a loop wire as described by others in this thread so that you get the speaker sound sent back into a sound source from which to record
3. get down into the guts of some files and create a virtual loop in some Alsa config files, from which you can then record.
As an aside ...... how sure are you that the source of the sound you wish to record is in stereo?
Is it actually arriving at your PC in stereo? ..... in other words can you listen to it on headset or speakers in true stereo?