Hi, Adonsa -
Thank you for your suggestions, and I did try some of them - I shutdown the Parallels Desktop, then rebooted my MacBook Pro with no external devices attached.
Then, I brought up Parallels Desktop manually, ensured that the USB Advanced Settings were set so that USB3 was OFF, then booted Windows 10. After W10 came up, I attempted to plug in devices, starting first with the Sabrent hub (which is a simple 4-port and a short 6 inch / 15cm cable), then the first FTDI UART cable, then the Xonar U5 sound "card". At that point, Windows 10 performed about 6-8 USB bus resets but finally settled out. I then attempted to add a second FTDI UART cable, and got a complaint from Parallels 11 that it was unable to add the device; Problem Report 106897055 was then generated and submitted. Adding the third FTDI UART cable caused even more bus resets which never settled out.
I can workaround the problem by eliminating the hub and running one FTDI UART cable and the Xonar U5. I can even run with the FTDI plugged into the lefthand USB port on the MacBook Pro and with the Xonar plugged into the hub which in turn is plugged into the righthand USB port. All that proves is that some stuff CAN flow through the hub, but apparently not the FTDI UARTs. So, maybe a trip to the store for a powered USB2 hub may indeed be the answer.
At this point, I don't want to waste more time trying to set up Windows 7 X4, but I appreciate the suggestion. Testing this configuration under VMware didn't go so well, either, but they appear to be a different set of issues (such as an apparently infinite wait for a device driver to download and install...)
Thank You !