The solution is quite simple: install a third-party virtual midi driver. I have not yet found references, but it would seem that the wavetable is one the features that have been removed/changed in this latest Windows upgrade.
After upgrading to Windows 10, I discovered that the softwares I use to write music ( Musescore, for example), are no longer able to carry out the reproduction of the music scores using the GS Wavetable Windows virtual midi interface, that allows software to play midi sequences also without a dedicated hardware.
