During my early months with the RPi I did a lot of experimenting on improving audio quality on my setup, as the default method had unsatisfactory results (hiss, hum, crackles)
My test unit then consist of
a. RPi3B, 2.4GHz wireless keyboard and mouse, 7" monitor
b. DIY Stereo Amp based on NJM2073D audio amp IC (quite sensitive to power line noise). connected to RPi3B's audio jack
c. DIY Setero audio VU meter based on LM3915 IC (still in breadboard setup), connected in parallel with audio amp's audio input
d. single power supply 5v 3.0A, supplying the 3 above (except for the monitor)
e. a pair of small 1w 8ohm speaker in make-shift cardboard baffles
f. OS was Raspbian Jessie with Pixel.
So I tested some of my USB Audio Dongles
1. an old Sound Blaster XFi Go: Audio was better than with RPi's on-board audio, and yes, definitely stereo output
2. a cheap Pluggable "brand" USB audio dongle: very good results, not much different from Sound Blaster, stereo output
3. another cheapo no-name USB dongle "7.1 channel capable": same as with 2, but configured as stereo only.
confirmation of audio "separation" was done using
visually can be seen on the VU meter display.
settled on using the Pluggable audio dongle for the mean time...
For the best possible audio experience with the setup above I had to use separate 5v 1A power supply for the audio amp and VU meter combo.
Used the setup for about 3 months before I decided to upgrade further...