Well, I received my pi 3 b+ yesterday. I set it up with my HDMI monitor, so the audio was defaulted to play through it's speakers.
That didn't sound very good; although it was tolerable. I plugged my Koss SportaPro headphones into the monitor's output jack. Again, the sound was OK, but not great. Certainly not the quality I am used to listening to from the Behringer UMC202HD on my Windows or Ubuntu PC.
Next, I plugged the UMC202HD into one of the USB ports on the pi. It was recognized right off, so I set it to the default, but I did have to reboot in order for sound to begin playing through it. Once I did that, I was rewarded with the clean, loud sound that I am used to. Using only the simple player "LXMusic simple music player", the audio was clean and loud - just as it is from my PC. I didn't have any issues with playing any of the .ogg files I loaded.
So I have made my decision: I need a dedicated sound system for the pi.
I am leaning on going with an add-on "hat" vs another USB device (or sharing the Behringer UMC) because it wouldn't use up the last USB port on the pi. I am already using the other 3 for keyboard, mouse, and a 128GB USB flash drive. I may or may not want to keep that flash drive connected, as I did buy the 64GB SD card for the pi, so there is plenty of space for music files and other stuff I do with the pi. That said, I have not yet made a decision on the audio system. I think I want input as well as output, so I will require more than just a simple stereo output.
I have decided that I do not want to share the Behringer UMC between the PC and the pi.
I am going to do a lot more research on this before making any decision.
If anyone has any recommendations for the add-on stereo in/out board (I have looked at both the HiFi Berry (no input) and the "Audio Injector" stereo sound card (both in and out), I would appreciate your input.